Joseph Smith (Jason Statham) is an ex-Special Forces veteran who went AWOL from his unit in Afghanistan, and now lives as a homeless drunk in London. While fleeing thugs, he breaks into an apartment and discovers the owner is traveling for months. He begins to assume the life of the owner Damon, calling himself Joey Jones, and spends his time looking for Isabel (Victoria Bewick), who he was separated from in the streets. He finds a credit card in the apartment mail, and starts to improve himself.
Joey makes friends with Cristina (Agata Buzek), a nun who runs the soup kitchen, and she helps him with medicine and messages to Isabel. Joey demonstrates his fighting ability to the owner of a Chinese restaurant, and is hired by Mr. Choy (Benedict Wong) as a driver/thug, collecting extortion payments and delivering drugs. He saves some of the money he receives and begins to do charitable work, ordering pizzas for the soup kitchen and buying things for Cristina. The homeless begin calling him "Crazy Joe". He finds Dawn (Vicky McClure), his ex, and starts giving her money. He tells the neighbors that he is Damon's boyfriend.
Joey invites Cristina to dinner, a barbeque in the street behind the Chinese restaurant, and Cristina shows him a police photo of Isabel, dead. Joey is enraged, but Cristina is able to calm him, and encourages him to start an honest life. He finds the men who chased him away from Isabel and beats them, interrogating them about Isabel's murder. Joey meets Cristina at an art gallery, saying he has information about Isabel's killer. She gets a bit tipsy, and they start a relationship. She tells him about a ballet ticket she has for the same date as when Damon is due to return - October 1st.
The police learn of Joey's activities, and begin questioning Cristina. Joey finds her later, and they share life stories. She was raped by her gymnastics instructor in Warsaw and killed him. Cristina is ashamed of her relationship with him, and asks her mother superior (Ger Ryan) to be transferred to another mission in Sierra Leone. Joey continues his search for Isabel's killer, trading services with Mr. Choy's boss. He is given the keys to a truck with boxes containing refugees/slaves to be delivered.
Meanwhile, Isabel's killer, Max Forrester (Christian Brassington) is beating a hooker. A neighbor finds his invitation to a rooftop cocktail party, and the invitation finds its way to Joey. The party is on October 1st.
Joey meets Cristina on October 1st. He asks her to take photos of him for his daughter, and he hints that he won't be the same for long (Damon is returning soon). She tells him she's leaving for Africa, and that she wants to be with him before she leaves. Damon returns, and they escape through the back. She invites him to the ballet, and Joey must now decide whether to be with the woman he loves or to avenge Isabel. He delivers the photos and a bag full of cash to his ex-wife and daughter.
Cristina waits for Joey, but he goes to the cocktail party instead. He finds Max Forrester and throws him off the roof. Cristina leaves the ballet, still looking for Joey. She finds him drunk and sleeping on the curb. He's remorseful, and tells her why he left his unit.
The next day, Cristina is leaving for Africa, and receives a note from Joey...he paid his debts with everyone and is homeless again. As he walks the streets, the police are closing in.