The film opens following a man as his plane lands at a private hangar and he makes his way to his mansion with his armed guard escort. The man changes into his swimwear and proceeds to get into his private pool to have a swim. As he swims he notices his watch at the bottom of the pool and swims down to retrieve it. An unknown man grabs the swimming victim and holds him under until he dies. The assassin, Arthur Bishop (Jason Statham), then uses the chaos that follows to take a careful route out of the mansion and to a nearby river where he jumps in and makes his escape. Back home, Bishop meets with Harry McKenna (Donald Sutherland) whom Bishop regards as a friend and mentor figure. Harry pays Bishop for his work in Colombia and they discuss Harry's son Steve McKenna (Ben Foster) before parting ways.
Returning to his house, Bishop checks for his new contract only to find that it is to kill Harry. Bishop phones his employer to verify that the contract is correct and upon confirmation he requests a face to face meeting. Bishop meets with Dean (Tony Goldwyn) who tells him about a failed mission in South Africa in which assassins of the agency Bishop works for were killed. Dean tells Bishop that only two people knew about the mission, Dean and Harry, and that Harry had received money in exchange for the contract details. Bishop reluctantly kills Harry and makes it look like a carjacking. At Harry's grave Bishop meets Harry's son, Steve. Steve tells Bishop that he's going to go out and find any carjacker and kill them as revenge. Bishop secretly follows Steve and interrupts him killing a carjacker that he had run into. Bishop recognizes the raw potential of Steve and decides to train him as a "Mechanic". He adopts a chihuahua and instructs Steve to take the dog with him to a coffee shop each day at the same time. As Steve settles in to his routine Bishop escalates his training by taking him on a contract to show him what it's like. After Bishop strangles the man with a belt and stages it to look like an auto-erotic asphyxiation accident, he shows Steve all the planning that went into that assassination.
Bishop informs Steve that he has a contract of his own. The target is a mechanic for another agency named Burke (Jeff Chase), who frequents the same coffee shop Steve has been going to. Furthermore, Steve is told that Burke's only weaknesses are that he is interested in young men and small dogs (such as the chihuahua they adopted). Burke makes his move on Steve and invites him out to drinks. Bishop instructs Steve to slip a large dose of Rohypnol into Burke's drink to cause an overdose. Steve ignores this direction and instead goes with Burke to his apartment. As Burke begins to undress, Steve attempts to strangle him with a belt as he had seen Bishop do. They engage in a struggle, and Steve manages to kill Burke after much effort. Bishop is later contacted by Dean, who expresses his disapproval of Bishop's use of Steve for the Burke contract. Bishop replies that he was given that contract through Harry and not Dean. Angry at his indignation, Dean informs Bishop that he's on a short leash.
Bishop is given a new contract to kill the leader of a cult-like church, Andrew Vaughn (John McConnell). Steve and Bishop plan to inject Vaughn with adrenaline to simulate a heart attack, for which the paramedics would unknowingly administer a fatal second dose of adrenaline. As Bishop and Steve wait in the walls of Vaughn's hotel room, a doctor arrives and sets Vaughn up with an IV of ketamine. Realizing that the adrenaline overdose would be inhibited by the ketamine, they improvise and quickly suffocate him. When Vaughn is discovered by his guards, Bishop and Steve are found hiding in the walls of the hotel and are forced into a shoot-out with the guards. As the building is being evacuated, Bishop and Steve slip out and Bishop decides they should fly home separately.
At the airport Bishop sees one of the men he was told was killed on the South African mission that Harry had allegedly sold out. During a confrontation with the man, Bishop realizes that his boss, Dean, had tricked him into killing Harry and that it had been Dean who engineered the failed South African mission to cover up his own shady dealings. Having been misled, Bishop begins to get things in order, only to be ambushed by a group of mechanics. After taking them out, he uses one of their phones and discovers that Dean was behind the hit. Racing home, Bishop calls Steve only to find that Steve has also been ambushed at Bishop's house. Bishop directs Steve to a hidden gun, which Steve uses to kill his ambushers. Steve is waiting for directions when Bishop arrives. Bishop has Steve gather supplies for their new mission, while Bishop prepares to find how to get to Dean. In the process, Steve finds his father's gun, and realizes that Bishop had killed his father and not carjackers.
After working together to kill Dean, they decide to go to a gas station and during the way to the gas station, Bishop notice Harry's gun in Steve's jacket and realizes Steve has discovered the truth. Steve gets out to put gas in the car and adjusts the nozzle so it is pouring on the ground instead of in the tank. With Bishop still in the truck, Steve pulls out Harry's gun and shoots the gas, blowing up Bishop's vehicle and the gas station. Steve then returns to Bishop's house and plays a record on the turntable before going out to the garage and taking the vintage 1966 Jaguar E-Type coupe Bishop had been working on. As he is driving off, Steve notices a note on the seat of the car, which reads: "Steve, if you're reading this then you're dead!" Steve laughs and moments later the car explodes killing him. At the same time the record player has finished playing at Bishop's house and it activates a tripwire, which causes Bishop's house to also explode. Back at the gas station, a security video reveales that Bishop escaped from his truck moments before Steve blew it up, saving him. Bishop gets in another truck he had by the beach and drives away.