The movie begins with a preface - Charlestown, MA has the greatest number of robberies per person in the entire United States of America.
A well-rehearsed, four-man team of bank robbers, wearing hooded skeleton masks, watches as an armored car makes its scheduled pick-up at Cambridge bank in Boston. They storm the building, order everyone on the ground and empty the tills. They zip-tie the doors shut. One of the men takes Bank Manager Claire Keesey (Rebecca Hall) to the bank's vault. The two wait for the vault's automatic timer to engage, and Claire nervously puts in the combination. Claire spots a "Fighting Irish" tattoo peaking out from under one of the men's mask, but says nothing. They empty the vault, separate out the bundles with dye packs, bleach down every surface, and stick the surveillance footage hard drives in a microwave. One of the robbers, monitoring the police on a scanner hears a silent alarm coming from the bank. He angrily assaults the other bank manager who triggered the alarm, leaving him beaten and unconscious. Frantically one of the robbers grabs Claire, blindfolds her and takes her hostage.
The four man team drives Claire around in circles, stops at the beach, lets her out and tells her to keep walking until she feels the water on her toes. The men switch cars and torch the getaway van.
FBI agent Adam Frawley (Jon Hamm) interviews the still shocked Claire, who tells him that she saw nothing.
Back in Charlestown, the four robbers - Doug MacRay (Ben Affleck), James Coughlin (Jeremy Renner), Desmond Elden (Owen Burke), and Albert Magloan (Slaine) meet inconspicuously at the local, dilapidated ice rink. One of the men produces Claire's driver's license, and they all realize that she only lives four blocks away, and she may blow their cover. Doug meets with on-again off-again girlfriend Krista (Blake Lively), James' sister, who suffers from a drug problem. She claims that her daughter is Doug's, while Doug, and most everyone else, knows she isn't.
Doug tails Claire, following her to the local laundromat, and finds her crying, still in shock from the robbery. Despite the fact that he might blow his cover, he comforts her and asks her out. Claire tells Doug that the only thing she saw during the robbery was the fighting Irish tattoo, which belongs to James Coughlin, and could probably identify her captors if only she could hear their voices. Doug laughs nervously. James, who saw Doug's SUV parked around the corner is surprised to find Doug eating lunch with Claire. James rudely introduces himself to her, enraging Doug, as his tattoo is in full-view. Doug manages to cover the tattoo with his hand, before Claire can spot it, and James leaves. As Claire and Doug's relationship grows we learn that Doug grew up in Charlestown, was drafted by the NHL, but later returned to help run the family business. He lets it slip that his mother left his father and him when he was just a boy, and that he spent days searching for her, while his father simply cried. He supposes that she lives in Tangerine, Florida because she had relatives there. He tells Claire that his father "doesn't get out much." Claire shares with Doug that a few days earlier, a couple of crack-heads started harassing her and stole her Prius. Doug enlists James' help, and the two find the druggies' apartment. They break through the door with a sledgehammer, savagely beat the man, and James shoots him twice in the leg. They order him to leave Charlestown.
FBI agent Adam Frawley continues his investigation of the Cambridge bank robbery. He learns that the only way the team could've known so much about the bank's operations was if they had a man who worked for the local telephone company. They discover that one of the four men, Desmond, works for "VeriCom" and called in sick on the day of the robbery. Frawley also finds connections between this heist and a local kingpin nicknamed the Florist (Pete Postlethwaite) who seemingly orchestrates these robberies from the comfort of his flower-shop. Doug is hesitant to take on the next job, as the armored car's driver is highly trained and trigger-happy.
Doug visits his father (Chris Cooper), a permanent resident of the local prison. Doug's father was also an armored truck robber in his day, but killed a pair of security guards, resulting in his incarceration. Doug tells his father that he is ready to leave Charlestown, and furthermore asks why he never looked for his wife like Doug did. His father ends the conversation immediately and retreats to his cell, leaving Doug confused.
The team sets out for the next heist. They circle the block repeatedly, waiting for the typically punctual armored truck to arrive. When it finally does, the four disguise themselves with nun-masks and begin the assault. As they empty out the truck, the driver breaks protocol, exits the cab, and draws a gun on them. James shoots the driver, and the four take off, with the police in hot pursuit. The police chase the team all around Boston, but the team manages to switch cars and torch the getaway vehicle, leaving tons of false DNA in the van, which they procured at a barbershop. Frawley learns about the robbery-in-progress and orders the local PD to shut down the Charlestown bridge. The four men manage to beat the cops to the bridge and arrive in Charlestown. As they empty the contents of the van: money, guns, etc, all four are surprised to see a local cop, sitting in his patrol car across the street, watching them unload the money. The cop turns the other way, allowing the four men to make their escape, uncontested.
Enraged, Frawley orders the forensics team examining the vehicle to find a fingerprint. Frawley manages to find evidence enough to bring in Doug's team for questioning. All four men deny their involvement in the robberies, citing purchased alibis. Frawley promises Doug that he will spend the rest of his life in federal prison, but is forced to release him on lack of evidence.
Doug plans to leave Charlestown with Claire. The two spend more and more time together. Claire has quit her job and now divides her time between volunteering at the local youth center/hockey rink and a community garden. Doug purchases Claire an expensive necklace as a token of his love for her.
The Florist has found another heist for the crew, which Doug refuses to take. He tells his crew that he is out and will be leaving Boston with Claire. Doug goes to the Florist, hands him a stack of cash and tells him that he's retired, and that the other three men can pull off the heist by themselves. The Florist doesn't accept Doug's resignation, and threatens that if he leaves, Claire will suffer. The Florist then reveals that Doug's mother never ran away. Doug's father also wanted out, so the Florist sterilized him and hooked his mother on drugs. She died via overdose and Doug's father never told him. Doug threatens the Florist, and his gun-toting henchman, that if they touch Claire, they will both die. Doug accepts the job.
Agent Frawley, who has been listening to her phone calls, visits Claire. He reveals that Doug is the lead suspect in her robbery/kidnapping, and upon seeing the necklace; he advises that she get a lawyer. She calls Doug, who fears that the Florist has gotten to her, goes to her apartment, only to find that she now knows the truth. She kicks him out. Frawley then visits the local bar, finding Krista strung out and drunk. He reveals to her that Doug intends to leave Charlestown with another girl, which angers Krista, though, not enough to reveal the location of Doug's latest heist.
Doug and James arrive at Fenway Park, the site of the Florist's job. Doug plans the job from a nearby hotel room. Krista arrives, unexpectedly, with her daughter. She begs Doug to take her with him wherever he's going. Doug kicks her out, with her daughter, telling her that the kid isn't his. Krista leaves in a huff, and strung out on drugs, crashes her car with the kid in the backseat. She's taken to the hospital where she has her stomach pumped, and is visited by Agent Frawley. Frawley promises to provide her now foster-child a better life if Krista tells him the location of the next job. She tells him.
Doug and James, dressed as Boston cops, sneak into Fenway. They make their way to the money room and are confronted by Fenway security. Doug and James cuff the security staff and money-counters and load their duffle bags with cash. They stroll to the underground parking garage where they meet Albert and Desmond dressed as EMTs in the getaway vehicle - an ambulance. Doug and James change into similar uniforms and as they're about to escape, they discover that Frawley has the entire stadium surrounded by cops. A SWAT team enters the building and James shoots at them. A firefight breaks out. A pair of flash-bangs are tossed into the garage. Desmond, disoriented, stands to his feet and is shot in the head. Albert, now wounded, tells Doug and James to change into their cop uniforms, and that he will drive out in the ambulance, acting as a distraction for the other two. He bursts through the garage doors and is promptly shredded by gunfire, crashing the ambulance into a roadblock. Doug and James quickly don their cop uniforms and sunglasses, and manage to sneak out of Fenway amongst the chaos.
Frawley's men reach the money room, discovering the security staff tied up on the floor. They tell Frawley that cops tied them up. Frawley surveys the crowd, and finds a lone officer, James, carrying a large duffel bag. He orders James to surrender. James opens fires on Frawley, missing, and is ultimately gunned down as he crosses a busy street. Doug watches from afar, and steals a police cruiser. He drives directly to the Florists' shop, where he kills the Florist and his henchman.
Doug calls Claire, who frantically answers. She asks him to come over, but doesn't tell him about the two-dozen FBI agents in her apartment listening to the phone call. Unbeknownst to Claire, Doug is a few windows away watching her talk on the phone. Doug, feeling betrayed, agrees to meet Claire at her apartment at a certain time - moments before hanging up she tells Doug that she looks forward to seeing him and that "it will be like one of her sunny days" (a cryptic "Don't Come" message she passes to him). Doug smiles and heads back to his apartment. Donning a bus driver uniform, he steals a city bus, avoids police roadblocks and drives it to the local train station, where he boards a train headed to Miami. At Claire's apartment, Frawley considers the meaning of the "sunny days" phrase, and realizing he's been duped, calls off the stakeout and leaves.
A few days later, as Claire works in the community garden, she comes across a buried briefcase loaded with cash, a Tangerine, and a note from Doug. With that money Claire renovates the dilapidated ice rink and dedicates it to Doug's mother.
A now bearded Doug stares out over the bayou in Tangerine, Florida, all alone.
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