NOTE: This spoiler was sent in by BroadwayBabe.

During the opening credits, a man is shot as he walks to his car; a notebook that he carries is stolen from beside the body.  A man is led into an office and frisked; he then proceeds to stab both the guards with some kind of poisoned dart and kill the man behind the desk with a baseball to the eye.  He then takes the book in front of the man at the desk.  A young man walks out of a hotel and is shot by a sniper.  There is much screaming and general confusion.

The movie opens in a bus station.  A young man walks in and sits down, dropping his bag next to him on the floor.  There is an older woman asleep on the bench nearby.  The man dozes off and is awoken by Goodkat's (Bruce Willis) voice saying "There was a time."  He is in a wheelchair.  The guy mishears and tells him the time; Goodkat corrects him, tells him that once that old lady on the bench was beautiful...there was a time.  He tells the guy that he is in town for a Kansas City shuffle 20 years in the making, and begins to tell a story, which we see in flashback.  A doctor drugs a horse to make it a sure win in a horse race; there is some kind of new gang in town running a gambling ring.  Word gets around via bookies, and a waiter in a restaurant hears about the horse and tells his nephew, a man named Max.  Max takes his son, Henry, out to the racetrack on the pretense that they're going to a Mets game, and tells him to wait 15 minutes, giving him his watch to time him.  He goes in and places a bet of $20,000 on the horse--number seven in the tenth race.  The horse is leading, then falls down before the finish and loses the race.  Max is horrified, and walks back out to his car, which is nowhere in sight.  Two men approach him and ask for the money he bet before knocking him unconscious.  He awakes in a room, battered and bloody, and Goodkat explains that the new gang wanted to make an example of him.  They killed everyone: we see the dead bookie who took his bet, a man with a gun walking into the kitchen where Max's wife is washing dishes, and a man standing behind Henry with a gun pointed at him.  The men place a plastic bag over Max's head to suffocate him.

The guy in the bus station is shocked by the story.  He asks if that's a Kansas City shuffle; Goodkat says no, that's a Kansas City shuffle, pointing toward the old lady.  She is gone.  The guy looks back at the wheelchair, but Goodkat is no longer there; he is standing behind the guy, and breaks his neck.  He says, "Sorry kid, but for a Kansas City shuffle you gotta have a body."  He wheels the corpse out to a truck in the wheelchair.

We then see Slevin Kelevra (Josh Hartnett) wearing only a towel and cleaning his face after shaving.  There is a knock at the door and he goes to answer it.  In bursts Lindsey (Lucy Liu) who lives across the hall.  She stops when she sees him and says "You're not Nick."  Slevin explains that he is a friend of Nick's who is staying with him for a while.  Lindsey notices that his nose is swollen and bruised and asks about it.  Slevin tells her about his recent string of bad luck experiences: His apartment was condemned due to the presence of an exotic termite; he went to his girlfriend's place and let himself in with his key, only to find her having sex with another guy in the bedroom ("It was an accident!" she says lamely, to which he replies "What, like he tripped and you fell?"); so he calls Nick to come stay for a while and as he's walking through New York he is mugged by a guy who steals his wallet ! and breaks his nose.  Lindsey asks why the guy didn't take his nice watch or suitcase, and Slevin has no idea.  He tells her that the door was open when he arrived.  Lindsey is taken by the idea that something has happened to Nick and is thoroughly excited to play detective.  She has to go to work, and walks out the door.  Slevin starts to take off the towel, just as Lindsey bursts back in and sees him naked.  She leaves again as he covers up, then pops back in hoping to catch him again.  He is obviously flattered by this.

There is a knock at the door after Lindsey leaves and two thug-looking guys, Slow and Elvis, burst into the apartment.  They are looking for Nick Fisher, and think Slevin is him.  They tell him that the Boss wants to see him, and Slevin mouths off to them, causing Elvis to punch him in the stomach.  Elvis says that if he makes one more smart-ass comment, he'll break his nose; Slevin manages to croak that it's already broken.

Slevin is sitting in the back of a car, still in the towel, obviously having been punched in the nose again.  They pull up to an apartment building and ride the elevator up to the penthouse, where Slevin is sent to meet the Boss.  There is a big man standing by the window and Slevin starts to explain that he's not Nick Fisher, that he has no ID because he was mugged, when the real Boss (Morgan Freeman) walks down a staircase to talk to him.  He tells him the story of the Shmoo, a creature whose purpose is to be used by humans to eat and make things.  He tells Slevin--whom he constantly calls Fisher--that he is the Shmoo.  It comes out that Fisher owes the Boss $96,000.  We learn that the man who was shot in the opening scene is the Boss's bookie.  The Boss offers to cancel Fisher's debt in exchange for a favor.  He explains that the young man who was shot outside the hotel was his only son, and that he believes that the man responsible is the Rabbi. ("Why is he called the Rabbi?" Slevin asks; "Because he's a rabbi," the Boss replies.)  He tells him that in retaliation for this he plans to kill the Rabbi's son, Itzhak the fairy.  ("Why is he called the fairy?" "Because he's a fairy." "You mean he has wings, glittery dust, all that?" "He's a homosexual.")  He explains that in order to cancel the debt, Slevin is going to kill the Rabbi's son.  Slevin is horrified, but the Boss tells him to think it over.  As the elevator doors close on Slevin, the Boss says, "Let me get this straight: I'm paying you to kill someone, and you're hiring someone else to do the job?"  Goodkat steps out from behind a wall and assures him that he will kill someone.

Two cops parked outside take photos of Slevin as he leaves the Boss's headquarters.  Brikowski (Stanley Tucci) the lead cop tells the other one to run a check on the guy, because he's either in big trouble or bad news, and either one is interesting to them.

Slevin returns to the apartment when Lindsey returns to ask if Nick is back.  The phone rings but there is no answer when Slevin answers.  Lindsey dials *69 and finds out that the call came from a hotel.  She has a friend who works there, and says that she will check it out, then asks why Slevin is still wearing the towel.  He doesn't have time to answer as she has to get back to work.  He manages to get dressed before two Hasidic Jews knock on his door and tell him that the Rabbi wants to see him.  After another punch to the stomach, he is driven to the building across the street from the Boss's headquarters, up to the penthouse.  It is revealed that the Boss and the Rabbi once worked together and now are opponents.  Slevin meets the Rabbi (Ben Kingsley) who also thinks he is Nick Fisher and owes him $33,000.  The man killed with the baseball was the Rabbi's bookie.  He tells Slevin that he has three days to come up with his money.  Goodkat emerges from another room after Slevin leaves, and the Rabbi tells him that he knows when people are lying, and that is not Nick Fisher.  Goodkat says he knows, and that he and the kid have unfinished business.

Slevin gets back to the apartment and Lindsey returns.  She has spoken to a friend at a hotel that Nick got a call from and finds that the room is registered to a man named Smith.  As she is going to check out the room, the alleged Smith walks out of the room and she follows him into the elevator, getting a photo of him with her camera phone.  It's Goodkat; Slevin says he doesn't recognize the guy.  Lindsey also says that she followed Smith/Goodkat to an apartment building and saw Slevin leaving that same building with two Hasidic Jews on either side of him.  She asks him to explain everything, and he does (we just see a flash of all the events that have just transpired).  Lindsey believes that Nick set him up, even the guy who mugged him only took his ID so that he could be mistaken for Fisher and have no proof otherwise.  She asks Slevin what he's going to do about the Boss.

Cut to Slevin telling the Boss that he'll kill Itzhak.  He asks for a week; the Boss says three days.  They agree that whoever wins a game of chess chooses the time limit.  As they play, the Boss tells him about Itzhak's bodyguards and his alarm button on a Star of David necklace.  The Boss wins the game.  Slevin has three days.  During the conversation it switches from the Boss-Slevin conversation to an earlier one between the Boss and Goodkat, in which Goodkat says that in order to make it look like the Boss had no involvement, the kid should go in and have a romantic tryst with Itzhak; he will then shoot Itzhak, Goodkat will shoot Slevin, and it will look like a suicide pact.  The Boss approves of this idea, but Slevin is unaware of that part.

Brikowski, the cop from earlier, goes down to the morgue where Lindsey (who is a coroner) has the bodies of the Rabbi's bookie and his goons laid out.  Brikowski tells her that the guy was his bookie back in the day.

Lindsey and Slevin are out on a date at a nice restaurant.  They are flirting heavily, and Lindsey asks him if he comes there often.  He says no, but he knows of someone who does.  He points out Itzhak the fairy to her.  He also points out the bodyguards, and the cops from earlier are watching from the bar.  When Itzhak gets up to go to the bathroom, Slevin follows; so does Brikowski, and so do the bodyguards.  Slevin and Itzhak are inside and the door locks; Brikowski goes in after Itzhak leaves and interrogates Slevin, asking him who he is because he isn't Nick Fisher.  Slevin brushes him off.  He tells Lindsey as they're walking back that he has a date with Itzhak.  She tells him that he reminds her of James Bond.  As they get up to the apartment building they are opening their respective doors (where Slevin got a key I do not know) but he winds up following Lindsey into her apartment and, subsequently, the bedroom.  As they are cuddling in bed there is a cute moment where they discuss which James Bond she meant, eventually setting on Sean Connery as the best.

The next morning Slevin goes out to get coffee for them before Lindsey goes to work.  The Boss and the Rabbi both have people watching him.  Brikowski and the cops grab him and pull him into their van to interrogate him, and he reveals that his name is Slevin Kelevra.  That's the only information he'll give up, and only after they beat him up a little.  He heads back to Lindsey's.  After Lindsey leaves the Boss's men come up to give him something to wear, and as they leave we see that the Rabbi's men have been shot dead.  Slevin goes to Itzhak's apartment, looking nervous.  Itzhak invites him in and tries to make him comfortable, but Slevin says, "Someone is trying to kill you."  When Itzhak asks who, Slevin replies, "Me," and shoots him in the stomach.  Goodkat walks up behind him with a gun.  Slevin hears him and turns around just as Goodkat fires, and hits Itzhak in the chest, finishing him off.  Slevin stands up and says "That was close."  Goodkat hands him a key and Slevin walks out to a parked SUV, pulling a body bag out of the trunk.  It's the guy from the bus station at the beginning.  While Slevin handles that, Goodkat notices a false wall in the room.  He presses the panic button on Itzhak's necklace and shoots the bodyguards as they break through.  Slevin dumps the guy's body next to Itzhak's and switches watches with him.  As Slevin and Goodkat walk away the apartment blows up.

Goodkat goes to meet with the Boss, who is angry that the job that wasn't supposed to look like a job is beginning to look very much like a job.  He laughs and decides it doesn't matter.  Slevin goes to meet with the Rabbi, carrying a suitcase that he says contains all that he owes him.  The Rabbi is reading the Torah and explaining that Saul didn't disconnect the ringer on the telephone, as they don't answer the phone on the Sabbath.  He doesn't understand.  The suitcase is revealed to be empty, and Slevin tells him that Saul and all the others are dead before knocking him unconscious.

The Rabbi wakes up in the Boss's headquarters.  He recognizes the place as he has been there before, and makes a nice little speech conceding defeat to the Boss.  The Boss replies that there is no need, and we see that they are both taped to chairs back to back.  They argue about fault, about the death of the Boss's son, and the Boss tells the Rabbi that Itzhak is dead.  Slevin walks down the stairs to address them.

Here the scenes cut around a bit.  We see Brikowski in the morgue with the charred body of the bus station guy, whom Lindsey cannot get ID for.  She does, however, recognize Slevin's watch and looks horrified when she realizes who he must be.  Goodkat steps in through the door of the morgue and shoots her in the chest.  She falls.

Brikowski goes to his patrol car, where he talks on the phone to a guy at the station who tells him that a new guy recognizes the name Slevin Kelevra.  The entire explanation is wedged in between the next few shots.

Slevin asks the men if they remember the seventh horse, tenth race, of the 1979 horse race.  He recounts the story and how they were the ones who killed Max and his whole family.  He recalls how they had to hire a specialist to take out the kid, and the specialist is Goodkat.  In flashback, Henry turns around and looks at Goodkat, who chooses not to shoot him and instead puts him in the car.  The Boss and the Rabbi are stunned to learn that Slevin is Henry.  He puts plastic bags over their head like they did to his father and tapes them into them so that they suffocate.

Brikowski is listening to the explanation from the man at the police station, learning that Slevin is the name of the horse from that race.  He also adds that the new guy is Jewish, and that the name Kelevra is Hebrew.  He says "It means..." as Slevin sits up in the backseat, puts a gun to Brikowski's head, and finished the sentence with "Bad dog" before shooting him.  In the flashback we see that the man who shot Max's wife was Brikowski.

Slevin is sitting at the bus station alone when Lindsey walks in.  He goes over and hugs her.  We see that on the morning he returned with the coffee, Goodkat tells him that Lindsey has to die because she took his photo.  Slevin agrees, then goes in and tells Lindsey where he will shoot her and how to protect herself.  When she fell in the morgue, she was wearing a bulletproof vest and a packet of blood to make it look authentic.  Back in the bus station they are embracing and Goodkat walks up.  Slevin tells him that he didn't think he'd understand, but Goodkat says he does.  He gives Slevin back the watch, which is shown to be the watch Max gave Henry at the racetrack.  Goodkat walks away, and in a final flashback Henry is shown in the backseat of Goodkat's car saying he wants to go home.  Goodkat replies that neither of them will be going home for a long time, and turns on the radio to hear the song "Kansas City Shuffle" playing.

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Everything that has happened in the film has been a setup, planned by Slevin (Josh Harnett) and Mr Goodkat (Bruce Willis).

Slevin is Henry, the young boy we saw at the start of the film. His father and mother were killed on orders of the Boss (Morgan Freeman) and the Rabbi (Sir Ben Kingsley), as Henry's father lost a lot of money on a crooked horse race. Mr Goodkat is called in to kill Henry, who is only seven, but takes pity on him and we are lead to believe that he raises him as a son. 20 years later, Slevin comes into New York and kills the son of the Boss. The Boss then contacted Mr Goodkat in order to kill the son of the Rabbi in retaliation, as he believes it was the Rabbi who ordered his son's shooting. Slevin shoots the Rabbi's son and Mr Goodkat deals with his bodyguards. Then the two of them kill all the Boss's and Rabbi's enforcers and tie the two of them up in the Bosses office. Slevin kills both of them the same way they killed his father; a plastic bag, gaffa taped over their heads. Slevin also kills Detective Brikowski (Stanley Tucci) as he was the one who shot his mother, because (it's implied) he owed money to the Rabbi.

Whilst this is happening, Mr Goodkat shoots Lyndsey the pathologist (Lucy Liu) because she took a photo of his face when they were in the lift together. However Mr Goodkat told Slevin he would do this, and Slevin tells Lyndsey to wear a bullet proof vest. Slevin meets up with her at the airport, but so does Mr Goodkat. But he lets them go, showing the same mercy he did to Slevin 20 years ago..