Four lifelong friends from the Boston neighborhood of Charlestown, Douglas “Doug” MacRay, James “Jem” Coughlin, Albert “Gloansy” MacGloan, and Desmond “Dez” Elden, rob a bank. They take the assistant manager, Claire Keesey, hostage, but release her unharmed. When they find out Claire lives in their neighborhood, Doug begins to follow her to find out how much she has told the police, and to make sure that Jem does not eliminate her as a witness. Soon a romance grows between them, which Doug hides from the gang. As they grow closer, Doug tells Claire of his search for his long-lost mother, who he believes went to live with his aunt in Tangerine, Florida. He also recounts his chance to be a professional hockey player which he threw away for a life of crime, following in his father’s footsteps. She in turn tells Doug that she saw a tattoo on one of the robbers, and he realizes that she can identify Jem and send them all to prison. He knows that Jem will kill her if he discovers the truth, so in order to discourage her, he advises her to tell the police, and tells her that the authorities will then put her in witness protection and send her to live in another state. His plan works, and she decides not to tell the police.
FBI Agent Adam Frawley surveils the gang and recognizes their ties to local crime lord Fergus “Fergie” Colm, who has another robbery planned for them. During a visit to his father, Stephen, in prison, Doug reveals his plans to leave Boston and go to Florida. The gang’s next robbery in the North End goes awry, and the gang barely escapes. They are brought in and interrogated by Frawley, but he fails to get any confessions and is forced to release them.
Doug asks Claire if she will go away with him, and she agrees. When Frawley learns that Claire quit her job, he wiretaps her phone, and threatens to prosecute her as an accomplice after realizing that she is seeing Doug. Shocked to discover that her lover was one of her assailants, she is forced to cooperate with the FBI and breaks up with him. Meanwhile, Fergie threatens to kill Claire if Doug does not go through with the job, then proceeds to tell Doug how he controlled his father by making his mother an addict, which led to her suicide. Doug gives in but swears that he will kill Fergie if anything happens to Claire.
At Fenway Park, Doug and Jem enter disguised as Boston police officers, steal $3,500,000 in gate cash, and prepare to escape in an ambulance, disguised as paramedics. Krista, Doug’s ex-girlfriend and Jem’s sister, threatened by Frawley and heartbroken by Doug’s going away without her, reveals enough for the FBI to surround Fenway before the gang can get out. The gang is caught in a firefight with FBI SWAT operators, and Dez and Gloansy are killed. Frawley spots Jem and they exchange gunfire. Jem, determined not to go back to prison and out of ammunition, commits suicide by cop by running out of cover with his guns unloaded and is killed. Knowing that Claire is in danger and that he will never escape as long as Fergie is alive, Doug murders Fergie and his bodyguard and calls Claire. Watching from across the street via binoculars, Doug sees that the FBI are with her as she tells him to come over. He, at first, thinks she means to betray him, but she gives a clue verbally, to warn him away. Doug flees, donning an MBTA uniform and escaping on a train. Frawley deduces that Claire tipped off Doug, but doesn’t arrest her because her choice of words was circumstantial.
Later, while gardening, Claire finds a buried bag containing the stolen money, a tangerine, and a note from Doug, suggesting that she can make better use of the money than he can, and that they might see each other again. Claire donates the money, in memory of Doug’s mother, to refurbish the local ice hockey arena where Doug once played. From the deck of a small house, Doug looks out over the water, forlorn, but seemingly safe in Florida.