Boston, 1978Two Irish-American brothers, the heroin addicted Stevo (Sam Riley) and his older brother Bernie (Enzo Cilenti) are driving a truck on the way to meet with some guys. Stevo tells Bernie about a fight he got into with another guy the previous night due to an altercation with the other guy's cousin. Waiting for them outside of a warehouse are IRA guys Frank (Michael Smiley), Chris (Cillian Murphy), and their American mediator Justine (Brie Larson). Before the brothers show up, Chris asks Justine is she's FBI. She says she's "IIFM" - "in it for myself."The brothers arrive and meet with Chris and Frank while Justine waits in the car. They are all met by Ord (Armie Hammer), a rep for the arms dealer they are about to do business with. Ord makes some smart-ass comments to the other guys before bringing them inside the warehouse.The buyers meet with the dealer, a suave South African named Vernon (Sharlto Copley), and his right-hand man Martin (Babou Ceesay). They are joined by other associates, Gordon (Noah Taylor) and Harry (Jack Reynor). Vernon greets Justine and compliments her before saying she put on weight. He then presents Chris with assault rifles. Chris points out that the order was incorrect, as he ordered M-16's but received AR-17's. Vernon is not happy with how this is going, but Chris and Frank decide to take the weapons anyway and hand over their money in a briefcase. As the transaction happens, Chris tries to invite Justine out to dinner in front of the other guys. Justine appears disinterested in the offer.Stevo recognizes Harry as the guy who beat him up the previous night. He tries to avoid Harry, but Harry spots Stevo and immediately flips out. He tells everyone that Stevo tried to coerce Harry's cousin into a sexual act and then smashed a bottle over her face when she refused, prompting Harry to react violently. Frank takes Stevo aside and starts smacking him for what he did and then demands that he apologize to Harry. Stevo starts to apologize but then starts mocking Harry's cousin. Harry goes to his car and takes out a gun, which he uses to shoot Stevo in the shoulder. The groups then run in opposite directions and everyone begins pulling out guns and begins shooting at each other. Martin gets grazed by a bullet on his head and passes out, leaving the briefcase unsecured.In the midst of the gunfire, two snipers, Jimmy (Mark Monero) and Howie (Patrick Bergin), begin shooting at the buyers, although nobody knows where the extra gunfire is coming from. Jimmy gets shot and slowly dies while Bernie gets up and gets shot in the back by Vernon until he dies. Stevo breaks down and rushes to his brother's side, swearing vengeance on his killer. Howie then speaks up, and Ord recognizes his voice. Before Howie can tell Ord who sent him, he gets shot dead by someone in the warehouse.Vernon still wants to try and nab the briefcase. Chris requests that Vernon lay his guys off Justine. She tries to make her way to the briefcase, but she is being followed by Gordon. Justine also ends up getting shot in the leg by Harry and she ends up shooting him back in retaliation.The guys continue to shoot at each other. Somewhere in the warehouse, a phone starts ringing. Frank tries to go after the phone so he can call for backup. Vernon follows him as the shooting continues. Gordon attempts to kill Justine, but she manages to take him down first. Justine then slowly passes out from blood loss.Frank and Vernon try to make it up to the room with the phone. Vernon starts going after Frank. Frank locks himself in a room and pours kerosene on the floor and making it spill out. As Vernon gets close, Frank ignites the fluid and sets Vernon on fire. Vernon manages to extinguish the flames and kill Frank.As the shooting continues, Martin manages to regain consciousness. It turns out Jimmy and Howie were sent by him to steal the money on their own. Martin engages in the shootout, but he soon dies from his earlier head wound.Chris makes it upstairs and finds Vernon slumped against the wall. Vernon tries to talk Chris down, but Chris shoots Vernon in the face, killing him. Chris makes it to the room with the phone and starts to call for backup. The phone gets cut off by Ord. Meanwhile, Harry and Stevo are still going at each other.Chris's IRA associate Leary (Tom Davis) arrives to the warehouse. He encounters Harry and starts fighting with him. Harry gains the upper hand and beats Leary to death with a crowbar. Harry grabs the briefcase and hops into his van. Stevo continues shooting at him as he lies wounded on the ground. Harry drives the van over Stevo's head, but Stevo manages to shoot Harry through the ass, killing him and causing him to crash the van.The fire that caught Vernon earlier sets off sprinklers. Only Chris and Ord are left standing. Chris takes the briefcase out of Harry's van. He and Ord decide to just call a truce before the cops show up. Suddenly, a bullet goes through Ord's head and Chris's chest. Justine is still standing, and she takes the briefcase. As Chris is slowly dying, he expresses regret over not getting to take Justine out to dinner. She agrees and leaves him. Chris dies. Justine walks to the front of the warehouse using a rifle as a crutch for her wounded leg. Then, the police show up and presumably arrest Justine.