NOTE: This spoiler sent in by Steve K.

The movie starts with a man called Theo (Clive Owen) in a coffee shop. People are gathered around a TV, where the news reports that the worlds youngest person, known as "Baby Diego", was stabbed to death at age 18 after spitting in the face of a autograph-seeker. Many seem sad but Theo doesn't seem to care; he exits the shop and walks down the street. He then stops to add alcohol into his coffee.  A few seconds later, the shop explodes and a woman is seen screaming hysterically while holding her severed arm.

Theo arrives to work where many of his co-workers are watching news obituaries for Baby Diego. Theo goes to his boss and says that Diego's death has affected him greatly and he would like to work from home today. We see Theo on a train, where a TV ad runs showing many famous cities in the world in flames. The ad them boasts that Great Britain is the only civilized power left. Theo gets off at his station and walks past cages filled with illegal immigrants who are being guarded by heavily armed army officers. All throughout the movie there are billboards and signs and ads telling Britons to turn in illegal immigrants over to the police.

We then see that Theo is actually visiting a friend of his called Jasper, who lives in a hidden house in the forest. While driving Theo to his house form the train station, Jasper pulls a "pull my finger" fart joke on Theo. When they arrive at the house, we see newspaper clippings of articles that state that the human race has gone infertile and nobody can find a cure. We also see a clipping of an article about a woman journalist who was allegedly tortured. There is also a picture of Theo, a woman, and a baby. The woman is Jaspers wife, and she just sits there mostly motionless and speechless, most likely the result of torture.

Jasper and Theo relax in Jaspers house, where Jasper grows weed/cannibis/something you can smoke. Jasper tries to tell a joke where the premise is about the Human Project, which is an organization that strives to solve the mystery of humanity's infertility. Theo then spouts off about how he doesn't believe in the project, thus ruining the mood. Jasper tells Theo its just a joke; he finishes it and the two men share a laugh.

When Theo gets back into London, he is suddenly kidnapped and pushed into a van. When he is unhooded, he sees that he is being held in a room with several armed men and his Julian, who was the woman in the picture at Jaspers house. Julian orders them to untie Theo and then the two have a short walk. Julian has joined a group called the Fishes; its a militant group that fights the government and saves illegal immigrants. Theo accuses the Fishes of bombing the coffee shop earlier, but Julian denies that the Fishes were responsible. Julian then tells Theo the real reason they kidnapped him; they need him to use a connection he has to get them travel papers that would allow the holder to get to the cost. Theo is offered 5000 pounds to cooperate. He is then re-hooded and dropped back in the city.

Theo visits his cousin; while going through metal detectors we see that he carries a lighter, cigarettes, and a bottle of some sort of alcohol on his body. Theo's cousin is a collector of art, he seems to have saved the statue David (albeit with a broken leg) and Picasso's Guernica from the hysteria outside Britain. Theo then asks for a favor (the papers, under the premise that his girlfriend has a sick brother) and his cousin says he'll see what he can do. Theo then asks his cousin how he can be so calm and collect art when the world is going crazy; his cousin replies that all he does is not think about the bad things.

Later, a Fishes member called Luke gives Theo instructions on where to go in a bar. Theo then tells Luke that he was only able to get the kind of travel papers where Theo would have to accompany the paper-holder. He tells Luke that he'll go along if the Fishes throw a couple of thousand more pounds into the deal. Theo then barely catches the bus designated by the Fishes; he goes to the top deck and sits next to Julian. They have an argument about which one of them hasn't gotten over the death of their son. They get off at a stop, and then Julian tells Theo that they picked him because he's the only person she can trust. Julian reveals to Theo that the papers are for a girl that needs to be moved to the coast.

Theo then meets a black girl named Key and her companion. Luke, Theo, Julian, Key, and the other woman drive in a car, where Julian demonstrates a trick which involves Julian and Theo catching a ping-pong ball with their mouths. They are just about to kiss when a burning car rolls in their way. A mob then attacks their car, and while they try to back away a man on a motorcycle shoots Julian in the throat. They beat the mob, but are pulled over by police. Luke then pulls a gun and kills the two policemen that pulled them over. They lay Julian to rest in a forest, where Theo takes a swig of alcohol, then cries.

They drive to a barn, where Theo is given room and board. The members of the Fishes are there to elect a new leader, and they will return Theo to London tomorrow. Key then summon Theo to a barn, where she shows him that she is actually pregnant. Theo is obviously shocked. Key tells Theo that Julian told her to only trust Theo and nobody else. In a meeting, the Fishes reveal that they cannot go public with Key because she is an immigrant; the government would abduct the baby and cover up its true heritage and most likely kill them all.

Theo wakes up at night and looks out the window. He recognizes the two motorcyclist that shot Julian as they have just stopped at the house. Theo sneaks outside and toward a window; he discovers that Luke had ordered that Julian be killed as a way of keeping Key here instead of having her delivered to the Human Project, which was Julian's plan. Theo gets Key and her helper and they escape. The car they pick wont start, so they roll it down a hill (with Key inside already) and get in as Fishes members chase them, unwilling to shoot because Key is inside. The car starts and they escape.

Theo drives to the safest place he knows: Jaspers house. When he gets in, he sees a box of suicide pills on the floor and he thinks Jasper and his wife killed themselves, but Jasper wakes up and scares him. Jasper claims they use the pills in rat poison so the rats die peacefully. Jasper is also amazed to see a pregnant Key, and has no problem hosting them. As for getting to the coast, Jasper tells them that he had arranged that a police officer called Sid take them into a internment camp for illegal immigrants on the coast, so long as they give Sid a bag of contraband as payment. Jasper reminds Theo to call Sid a "fascist pig".

At night, Theo heads Jaspers telling Key and her companion the story of Julian and Theo. They had met at a protest and fallen in love. They had a son, Dylan, but Dylan had died in a flu pandemic in 2008. Later, Theo and Key talk a little; Key has no idea who the father is.

The next morning, Jaspers security cams catch Fishes members coming. Jasper packs Theo and Key and Key's helper into a car and have them leave. He then opens the box of suicide pills (its implied he had his wife take them I think). The Fishes members led by Luke arrive and search Jaspers house. They find evidence of Jaspers plan to have Theo and Key go to the internment camp. As Theo watches from afar, they ask Jasper how long ago Key left; Jasper responds by asking them to "pull my finger". They then shoot him in the arm and the leg, but Jasper asks them to pull his finger again. They then kill Jasper.

Theo and Key and Key's helper hole up in an old school while they wait for Sid. Key's helper reveals she was a nurse and recalls how pregnancies kept declining until there were no more new babies. Sid then arrives and drives to the internment camp gate, where they switch to a bus with other illegals and are driven into a camp. While on the bus, they pass by illegals being caged and beaten viciously by army soldiers. Keys water breaks, and when soldiers are inspecting the bus, Key's helper makes a commotion to cover up Key's cries of pain. She is punched and taken off the bus and hooded; the bus, with Theo and Key still inside, drives on.

Theo and Key arrive at the camp, which is like a ghetto of many immigrant factions (curiously, many residents in the camp are heavily armed). They meet a woman that speaks little english, and this woman give them a room, as this was part of Jaspers deal with Sid. Key is due, and Theo delivers the baby; its a girl. Theo and Key share a laugh as Key holds her new daughter.

The following day, the lady and Sid return and are amazed to see a newborn baby. Sid then tells them that Fishes members broke into the camp at night. Sid then says he saw Theo and Key's picture on TV; they re wanted by police (in connection with the officers that Luke shot and killed earlier). He then pulls a gun on them, telling them that either side would probably pay big for Key and the baby. As they walk, Sid is distracted and Theo tackles him, forcing him to shoot off all his bullets in the magazine. The non-english speaking woman then hits Sid with a club, and this gives her, Key, and Theo time to run. As Sid tries to chase, Theo hits him in the face with a brick.

The woman leads them to a house with her relatives; they treat the baby with affection and adoration. A man tells Theo that they can find a ship so they can meet the Human Project at the designated meeting point at a bouy off the coast. Meanwhile, sudden factional warfare breaks out between the immigrants and the government; soldiers start exterminating immigrants and shooting anybody they see.

Theo breaks out with Key, the woman, and their guide. As they're about to climb into the sewer, they're caught by a group of Fishes and Luke. Luke takes Key and the baby away; the guide is shot to death afterward, and Theo is next until soldiers open fire on the Fishes, killing most of them. Theo takes the chance to follow Luke. He sees that the remaining Fishes have run into a building, which is now under assault by a tank. Theo rushes inside, past many injured immigrants, most of whom seem not to be resisting. He finds Key crouching in a corner with a baby, while Luke is pinned down by gunfire on the other side of the room. Luke tells Theo that the boy needs to stay here as a political weapon; Theo responds that its a girl, and then takes Key and runs. Luke shoots at Theo, but then the tank blows up the room.

As Theo and Key run downstairs, everyone sees the baby in Keys arms and are shocked into silence. Some people try to gently touch the baby as they walk by. Soldiers pour into the building, but when they see the baby, they too are awed by the sight. Theo and Key and the baby walk out of the building with all the soldiers just watching the baby. Then someone inside the building shoots again, and the soldiers resume shooting and blowing up the building. Theo and Key run.

They meet up with the no-english woman from earlier and climb into the sewer where there's a small rowboat. Theo tells the woman to get in with them, but she says no and bids them goodbye. Theo rows out to the designated buoy, where Key wonders whether or not they made it in time. Theo says that there's no way the Human Project would leave without the baby. Fighter jets fly overhead, and explosions can be seen in the camp. Theo then reveals he's dying; one of Luke's shots actually did hit him. Key tells Theo that she'll name her child Dylan, regardless of the fact its a girl (previously she had said she wanted to name the child "Bazooka"). Theo slumps over and dies just as the Human Project ship comes out of the fog spots them.

The movie ends with the words "CHILDREN OF MEN" in white on a blank black screen.

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Julian’s (Julianne Moore) murder was arranged by Luke (Chiwetel Ejiofor), who wants to use Kee’s baby as a tool for immigrant rights. He spends the rest of the movie chasing Theo (Clive Owen), Kee (Claire-Hope Ashitey) and Kee’s midwife Miriam (Pam Ferris). In tracking down Theo, Luke murders Jasper (Michael Caine). Kee delivers her baby in a refugee concentration camp and Theo brings her to the buoy where the Human Project ship meets her. Theo dies of his wounds before the ship arrives.