NOTE: Another great spoiler was sent in by William who says..."I'd just like to say this is one of the most superb dramas I've seen this year, with terrific performances from all of the primary players. Definitely worth checking out!".

The film opens with the rabid ranting and raving of Arthur Edens (Tom Wilkinson). Most of this speech is rambling and babbling. Essentially he begins talking about how one day he was leaving work and he walked out into the street into traffic and suddenly he found he could not move, even though there was traffic incoming. Essentially he tells us he was frozen in a moment of utter consternation brought on by the shambles he has made of his life doing what it is that he does. His vocation of acting as a legal attorney helping a corrupt company defend the indefensible has brought him to a point where he can no longer go forward and he must break the shackles with his old life and try something new. As we are listening to this ranting we are following the camera into an office where we see a legal team of dozens of people diligently working away on various operations such as taking phone calls, shedding documents, comparing notes etc. The camera comes upon a lawyer talking on the phone to a reporter about a story. The lawyer then walks over to Marty Bach (Sydney Pollack) and hands him the phone. He tells Marty that it’s a reporter from the Wall Street Journal inquiring about the "settlement". Marty looks infuriated but takes the phone. He simply tells the reporter that "U-North is not settling at this time" and that nothing has changed in the legal case. The reporter remains dubious, but Marty says he does not care and simply hangs up the phone. Marty then asks about where Karen went to. We then see Karen Crowder (Tilda Swinton) in the bathroom having what looks like a panic attack.

The film cuts to a card game where we see Michael Clayton (George Clooney) at the table with a handful of other men. One of the men recognizes Mike. He tells Mike he knows him from a game they played some years back. Clayton looks up and admits he recognizes him. The player says he won a whole bunch of money off Mike. The player continues to brag about winning a whole bunch of cash off Clayton and it clearly annoys him. Clayton says "I see you’ve bought some hair since then", but the other gambler retorts "yeah with your money". Clayton finally leaves the table and takes off.

Clayton is sent by a legal friend of his to the house of a man who desperately needs legal counsel. When Clayton arrives we learn the of the situation. Mike is talking to a man and his wife regarding the husband having just hit a pedestrian and possibly killed him that night. Mike explains to the man that his options are limited. That even though it may have been just an accident there are all sorts of variables that could affect the criminality of the case. Mike tells the man he needs to find a good trial lawyer for this, because he is going to need one. The man becomes irate and asks "What the hell are you supposed to be". He wants to know why Mike can’t be his lawyer and he tells Mike that he was promised a "miracle worker". Mike tells him that is not the case. He explains what he hoes by calling himself a "janitor". Essentially Michael Clayton is a overall strategic legal advisor. He has a tremendous acumen for understanding how to plan and execute a legal defense from start to finish even though he actually does not lead any of it. He merely advises. Clayton finally realizes there is nothing more he can give the man but advice and leaves.

As Clayton is driving on the road he notices his car’s GPS system is having various aberration's and glitches. Mike simply ignores these interruptions and keeps on driving. As Mike drives along the road a intriguing sight catches his eye. He sees three horses standing on a hilltop together be themselves. Mike is so captivated by this event he emerges from his car and goes over to the trio. Mike approaches the horses and seems fascinated by the fact that they are out here together all along in the middle of nowhere. However, suddenly Mike’s car explodes behind him and the horses go charging off. Mike is utterly stunned and is petrified for a few moments as he looks at the flaming wreck. When he finally regains his wits we see him begin running away.
The film now cuts to four days earlier ......

We see a kid getting ready to go to school. This is Mike Clayton’s son Henry (Austin Williams). Clayton is divorces from his wife and his son lives with her and his step-dad. Henry leaves the house and joins Mike in his car. Mike drives Henry to school. On the way Henry tells Mike about all the fantasy novels he likes to read. He describes how the current one is about a bunch of character’s who are in a universe where they each have a nemesis, but don’t know who that person is so they can’t trust anyone and just live for themselves. Mike laughs and says "that sounds familiar", obviously alluding to his own legal work.

We next see Karen Crowder, who is a head executive for U-North, which is a tremendous corporate agricultural conglomerate which creates various types of substances to help better cultivate land for farming. As we see her the film cuts back and forth between her giving an interview on camera with her boss Don Jefferies (Ken Howard) and Karen anxiously rehearsing the interview in her hotel room as she tries to prepare herself for it. The subject of Karen’s interview is about how she is taking Don’s old job as one of the head corporate leaders. As she is giving the interview a assistant walks in and interrupts them with what clearly appears to be a urgent matter. She tells them that Arthur Edens just stripped down in the middle of a deposition for their current legal battle. This is a dire matter, because Arthur is handling the legal defense for this case involving a weed killer which is potentially toxic and lethal which stands to lose U-North billions of dollars.

We then cut back to Mike Clayton who is visiting an older man. The man is a "Bookmaker" who takes bets and handles local gambling operations. It is here we learn of Clayton’s severe addiction to gambling. Mike is in debt for a massive amount of money to the gamblers. He has sold off much of his possessions, but yet still is approximately $80,000 short of what he needs to settle his debt. The man tells Clayton that he had better pay up in a week or the consequences could be catastrophic.

Mike then gets in contact with the head of his legal firm, Marty Bach. Marty informs Mike of the situation with Arthur Eden who also works for the firm Marty tells him of how disastrous what the ramifications of the Arthur’s mental breakdown are. Mike says that Arthur’s episode is understandable given the amount of time and energy Arthur has invested into this case. He basically says that for the last decade Arthur has basically "lived" this case and his life is tumultuous and fractured because of it. This case is in many ways just as big of a deal for the legal firm as it is for U-North, because it coincides with a vital impending merger with a London based firm. Mike then agrees to head to Minnesota to meet with Arthur who is currently being held in a police station.

As we see Clayton arriving at the station we hear Karen ( who is at one of the U-North offices) discussing with some her assistants Clayton’s background. She learns from documents that Mike used to be a major criminal prosecutor, but now has been seemingly relegated to a more menial task of handling "wills and trusts". Karen seems baffled that Michael is the one being sent to handle the crisis with Arthur. As she has been saying all this, we have been watching Mike arrive at the station and being sent to see Arthur is his holding cell. He finally meets with Arthur and it is here that we begin hearing more of Arthur’s crazed rambling from the start of the film. As Arthur finishes his lengthy and deranged claims Mike becomes completely livid and says "You stopped taking your pills again? Didn’t you?", as he shows him the still clearly full bottle. Arthur says he is no longer going to be a slave to his medication and that he has has his eyes opened by a woman who is one of the plaintiffs in the lawsuit against U-North named Anna. She claims that Anna is a beautiful, kind, and graceful soul and that he refuses to assist in the defense of a product against her and all the other people involved in the law suit knowing that it has really is a killer and how much pain it has caused them. Mike can clearly see how grievous the situation is. Arthur is potentially infatuated with a one of the people he is supposed to be defending U-North from and may have caused irreparable damage to the suit in the process. He tries to remind Arthur of his responsibilities and what he means to the firm, but Arthur only responds to this by proclaiming "I am Shiva The God of Death!". Realizing that Arthur is incorrigible at this point and is causing a scene at the precinct which could potentially make things even worse than they are, Mike just gets Arthur to leave with him and return to his hotel room.

The next day Mike goes to meet with Karen who is ostensibly belligerent over all that has transpired with Arthur. Mike reassures Karen that all will be fine. He tells her that Arthur does have a history of mental illness, but that it has been largely regulated in recent years. He tells her that his last episode was several years ago and that it was not a major issue and Arthur was able to continue his war largely unhindered. Mike reminds Karen how hard U-North has been pushing him on this case and how he has been near singularly devoted to this case for a long time now. Karen however, seems full of antipathy over Clayton’s claims and says that U-North has been paying Mike’s legal firm a very lucrative sum for Arthur’s time. Mike simply reassures her that he will straighten Arthur out and everything will return to its previous status. Karen however, seems completely dubious about this.

We then see Karen back at the U-North office watching a videotape of Arthur’s episode from the deposition. Everything is going routinely until Arthur suddenly stands up and begins spouting a bunch of rhetoric in a obstreperous tone about how Anna was right and how wrong he was. He tells her he doesn’t want to continue this charade any longer. As he is saying all this he is stripping off all his clothes. The tape finally ends and we are informed that Arthur then followed Anna into the parking lot outside totally naked. Karen is deeply mortified and aghast as she realizes how damaging this tape could be to further legal proceedings. Especially given how Arthur seemingly admits to U-Norths guilt in the case.

The scene shifts back again to Arthur in the hotel room at night, who has managed to contact Clayton’s son Henry. Henry tells him about another new book he is reading called "Realm & Conquest". He says in this book the characters are experiencing visions of things that would seem too fantastic to possibly be real, but they challenge conventional opinions and choose to believe that these illusions are real and that they must believe in them regardless of what others deem to be true, Naturally Arthur hears this books plot and it compels him to do the same and keep believing what he has come view U-Notth as, a corrupt company attempt to insulate themself from their crimes.

In the morning Arthur tells Michael that he should go see Anna and he too will feel stimulated to stop trying to help U-North in perpetuating their lies. Mike tries to assuage Arthur and tells him to just take a shower so they can get ready to return to the legal firm in New York. However, Mike gets occupied for a second and leaves Arthur alone, When he comes back the bathroom door is locked. Mike can hear the shower running, but when he knocks on the door there is no response. Finally, Clayton kicks the door down to discover Arthur has fled. He goes over to the window and sees that Arthur has gone out one.

Back at U-North Karen has just learned of the new situation. We see her sitting at a desk looking over a document with an agitated expression. We see the document she is reading. It is an old scientific report from one of U-North’s research teams about the hazardous effects of exposure to the weed killer involved in the law suit. It clearly documents that prolonged exposure to the substance can lead to cancer. As we see Karen reading this document, she is contacting someone on the phone. It becomes immediately evident from the surreptitious tone of the discussion that this is some sort of private cabal of operatives that U-north uses to deal with problems in all sorts of nefarious ways.

Meanwhile, Mike goes back to New York to meet with Marty. Marty is absolutely incensed over what has transpired. He tells Mike that this affair could mean the end of the firm. He says that if the case goes under that U-North could potentially sue the firm for their legal incompetence and ruin them. With the merger coming the situation is even more grave. Marty asks Clayton why he just didn’t have Arthur committed in Minnesota and says that would’ve helped quell the problems for the time being until things could be ameliorated. Mike tells Marty that it is not that simple and that involuntary commitment to a mental facility is a very arduous procedure. Mary asks Clayton "Well don’t we have anyone who knows a lot about these types of commitment cases?". Mike says yes, but informs Marty that the person who is the expert unfortunately is Arthur.

Next we see Arthur who has also returned to New York and is now roaming freely. He is living a reclusive lifestyle in a private apartment. However, the men hired by U-North are already watching him. They are tracking his movements around the city, have bugged his phone and are taking surveillance of his actions. All the while they are keeping Karen abreast of important developments.

During this time Michael goes and pays a visit to Anna in an attempt to see if he can gain more information about the case. He arrives at a farm back in Minnesota. After introducing himself to the family he taken to meet the woman with whom Arthur has become so enamored Anna (Merritt Wever). Mike gains some more information about Arthur and the case. She seems puzzled as to why Arthur seems to have selected to communicate with her when there are over four hundred other people suing U-North. Anna seems rather oblivious of how personally connected Arthur is to her.

Soon thereafter Arthur again contacts Anna over the telephone. He hints to her that he has more information about her case and he will contact her again in the near future about the matter. Anna still seems perplexed and feels awkward about being contacted by the man formerly leading the U-North defense regarding assisting her.

Back in New York Marty notifies Mike about Arthur getting in contact with Anna again. It has now become clear that Arthur still wont abandon this matter and he sends Mike to go to Arthur’s location and try to reason with him one last time.

Mike comes up on Arthur on the street. Arthur seems startled, but greets Clayton. Clayton however, is in no mood for pleasantries and is still enraged over Arthur’s continuing erratic and irrational behavior. Mike mentions about Arthur going to visit Anna again and how idiotic this was. Arthur immediately becomes suspicious of how Clayton could possibly have become privy to a private phone call he made to Anna. He inquires to Mike about this and Clayton responds that Marty told him. He then accuses U-North and the legal firm of colluding together to tap his phone. Clayton tells Arthur this is absolutely preposterous and says the most logical answer is that Anna contacted the police herself after Arthur called her. Arthur disputes this saying that Anna would never betray him like that, but Michael tells Arthur this is lunacy. He says after the crazy antics Arthur pulled at the deposition, stripping down before Anna and shouting all sorts of psychotic things, it would only be expected she would contact the police. Arthur as previously seems totally innocuous to Clayton’s continuing reproach and his attempts to reason with him.Finally Clayton makes threats about the state of Arthur’s sanity and how his recent behavior could lead to him being hospitalized, but Arthur still clearly retains his knowledge of legal matters. He tells Mike that this is New York and here he has no criminal record or history of threatening behavior both of which are critical for compulsory treatment. He says that now that he is out of Minnesota, that Mike can no longer use prior incidents there to try and go after him in this state. Mike finally concedes that he cannot win here and leaves.

Later that night Arthur is in his apartment. He makes a tape of one of U-North’s commercials. He then plays this tape as he makes a phone call, correctly knowing that the U-North agents are listening. He plays the commercial and then begins reading in a sardonic tone to them the scientific report from earlier in the film. As he reads it, he mocks the company and the consequences the report could have. The agents are shocked to learn that Arthur has this document in his possession. Once Arthur is done they meet with Karen. When she hears the tape she becomes very perturbed. She asks the agents what can be done. They make some suggestions about additional measures, but then Karen subtly implies she wants them to go much further than just mere subterfuge.

Following this we see Michael Clayton at a police station. He is being notified about Arthur’s death, The police detective Clayton is speaking with informs him that it look like a possible suicide or accident with some pills. Clayton seems confounded and doesn’t seem to believe that Arthur would be the type to kill himself, but the policemen reminds him about Arthur’s perverse behavior in recent weeks. Mike asks if he can go to Arthur’s apartment, but the officer says this is impossible and the place has been sealed. Mike later meets with a gathering of his fellow legal employees. There he tells Marty there is something more that Arthur was onto and he feels it was linked to his death. Mike also notified Marty about he desperately needs a loan. Marty promises Mike he will look into the matter and says given what a valuable asset that Mike has been to the firm he should be able to convince them to help him out. Marty also notifies Mike that Arthur was scheduled to meet with Anna in New York.

After contacting Anna’s sister and learning of Anna’s location Clayton arrives at a hotel where she is staying. Mike inquires with Anna about the final conversation she and Arthur had. Anna says Arthur told her he has an item in his possession which could win the case for her. Mike now becomes more convinced Arthur’s death was no mere suicide.

Mike goes to a family gathering at his sister’s house. There we meet his brothers: Timmy Clayton (David Lansbury) and Raymond Clayton (Kevin Hagan). Mike’s brother Timmy is a recovering substance abuser and while at the house the two have a rather uncomfortable confrontation over this, which Mike’s son Henry witnesses. While at the party Mike also talks with his brother Ray (who is a policeman) about needing a possible favor regarding Arthur’s death. After the party, Mike talks with Henry and says that his son doesn’t have to worry about going astray in life like Timmy did and that he believes in him.

Next we see Clayton meeting with his brother Ray again who gets him the keys for Arthur’s apartment. Ray tells Mike how serious this is and that he cannot get caught. Mike then heads to Arthur’s apartment, but he doesn’t know that the U-North guys are watching the place and they see him go in. Inside Mike looks around Arthur’s apartment. He finds a copy of "Realm and Conquest", the book Mike’s son Henry was reading. As Mike opens it a slip falls out that was being used as a page holder. The slip is a receipt from a copy story which clearly shows Arthur had been printing out a large number of copies of a certain document, However, shortly after this the cops show up and they arrest Clayton.

Raymond gets Mike out of jail, but is outraged regarding the effect this could have on his career as a policeman if they connect Mike’s entry into Arthur’s apartment to Ray. Mike tells Ray someone set him up at Arthur’s place and that they called the cops. Following this Mike goes to the printing place with Arthur’s receipt. There he finds out what Arthur has been printing so many copies of: the confidential U-North report. He tells the clerk to hold onto the documents for him. However, as he leaves the U-North operatives arrive and they learn about the copies of the document. They then contact Karen about this. She realizes she must do something immediately before Clayton finishes what Arthur started.

Mike goes to see Marty who gives him a check with the money needed to pay off his gambling debt. Mike then goes to see the man from earlier in the film so he can square off what he owes. From this point the film recaps the opening moments of the film. We see Clayton leaving the card game, but this time we also see the U-North agents putting an explosive bomb into his car. Since Clayton leaves the game early they don’t get to hook up the bomb in the correct manner. This is why Clayton was getting interruptions with his GPS signal earlier. However, Mike was not the only one having problems with the signal. The two U-North operatives were following him, but because of the intermittent issues with the GPS signal they keep losing track of him. After Clayton leaves the house of the man who was involved in the "hit and run" they lose Clayton and cannot seem to find him. This causes them to set off the bomb at the time when Mike is not in the car standing by the horses. So they GPS blunder inadvertently saves Mike’s life. We then see Mike run back to the flaming car and throw his personal belongings inside. After that we see Mike’s brother Timmy pick him up.

Next we see the staff at Clayton’s law firm being notified of his supposed demise. The film then cuts to Karen talking to a bunch of U-North chief executives about the conclusion of the law suit. She says the cost of settling will not be very severe and the company will be fine. Don Jefferies is there and he asks her to leave temporarily while they confer in private. Karen steps outside but is astounded to see Michael Clayton standing before her. Mike jokes saying that "Arthur is around here somewhere too!". He shows Karen the report and tells her how foolish she was to try and eliminate him. He tells her it would’ve been far easier and more prudent to just buy him off and tells Karen he wants 10 million dollars immediately. Don steps back outside and asks Karen to rejoin him. Clayton tells her to either pay up or he will go public with the report which he has in his hand. After trying to negotiate Karen finally realizes sh has no choice but to acquiesce to Clayton’s demands. However, when she finally agrees to give Clayton the money he promptly responds "You’re Fucked!". Karen seems confused and asks "you don’t want the money?". Mike then shows her a wire he is wearing which has been recording their conversation. Clayton then jokes and says "Wait let me get a picture of this!" and pulls out his cell phone to take one of her. He then begins to walk away. Don who has been listening to this becomes irate and calls for security to seize Clayton. We see a group of officers walk in, but they walk right past Mike and towards Karen and Don and tell them they are under arrest. Karen falls to the floor in dismay. We see Mike’s brother Ray is with the cops and Mike thanks him for helping him out. Clayton walks outside and gets in a taxi. He gives the driver some money and tells him to "just drive". As the taxi drives away the credits roll.

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Michael Clayton (George Clooney) is a legal advisor who works for a law firm representing U-North a biological farming conglomerate on a case involving a weed killer which kills. When the head lawyer the legal firm has on the case Arthur Edens (Tom Wilkinson) goes crazy the head of the legal firm, Marty Bach (Sydney Pollack) sends Clayton to try and fix the situation and control Arthur. When it becomes obvious he can’t and a potentially devastating research document might get released; Karen Crowder (Tilda Swinton) a U-North Executive, tries to contain the situation using a bunch of hired operatives. Eventually when Arthur plans to release the document they kill him. Michael Clayton eventually finds out about the document, but this in turn leads to him becoming the target of Karen and the U-North agents. After a failed attempt on his life, Mike confronts Karen and the head of U-North and has them arrested.