Paul Callow Mackenzie Crook accidentally runs a man over with his underground train after the man is pulled onto the tracks by his dog. After a week off he kills a second passenger who falls onto the tracks after having a heart attack.
Before taking time off for the second accident his colleagues tell him about a little-known London Underground rule: three 'under' within a month, and you lose your job — earning yourself ten years' salary in one lump sum. But being off for the next week means that Paul needs to find someone willing to die by the following Monday.
Paul sets about trying to find someone prepared to die by his train — and after hearing a report about Holborn Viaduct he comes across Tommy Cassidy. In a bar Paul explains to Tommy that he will pay him if he is willing to jump in front of his train. Tommy is scornful, asking what good the money will do if he has to die; Paul says that he can have one last weekend of fun. Tommy, however, wants to do something meaningful.
Tommy agrees to the proposal and decides to spend his last weekend making amends with his estranged family. He hires a car and the two travel to Liverpool. In the Lake District Tommy’s wife Rosemary Imelda Staunton and daughter Frances Gemma Arterton have moved there. The pair head north, and Tommy meets with resistance from his wife and daughter. He reconciles with her, who reveals she has found a new man. Paul meets the daughter at a local pub, and the two get drunk and sleep together.
On Sunday morning, Tommy tries to talk to his daughter and finds her in bed with Paul; he chases them over some hills before suffering a mild cardiac attack and being taken to hospital. Tommy and Paul then return to London.
Paul tells Tommy he does not want to kill him, but Tommy insists that he go through with it. On Monday morning, Paul is told by his colleagues that the “three and out” rule was a joke and there is no payoff. When the time arrives, Paul sees Tommy on the tracks and stops and reports an “animal on the tracks.” Then, crying and recalling > a deal's a deal, Paul accelerates toward Tommy, who recites William Butler Yeats' poem "The Lake Isle of Innisfree" as the train kills him.
Distraught, Paul stays at home and begins to write, changing the title of his novel to Three and Out. Frances receives a copy of the novel along with £10,000, with the dedication hidden behind the cheque: "In Memory of Tommy Cassidy." Realising her dad has died, Frankie breaks down in tears. She travels to London and meets Paul, and they go deep-sea diving with Great White Sharks, Tommy's last wish.
