Arthur Bishop, working as a “mechanic” (mafia slang for hitman), sneaks into the lavish home of a Colombian Drug Cartel leader and drowns him in his own pool. Upon returning home to Louisiana, he meets with friend and mentor Harry McKenna, who pays Bishop for his work.
Bishop is given a new assignment: kill Harry. Bishop’s employer confirms by phone that the contract is correct, whereupon he requests a face-to-face meeting. Dean tells him about a failed mission in South Africa, in which agency assassins were killed. Dean says that only himself and Harry knew the details of the mission, and that Harry had been paid for the contract details. Reluctantly Bishop kills Harry with the latter’s own gun and makes it look like a carjacking. At his funeral, Bishop meets Harry’s reckless son Steve. Bishop stops Steve from trying to kill a would-be carjacker in a misguided attempt at cathartic vengeance. Steve convinces Bishop to train him as a mechanic. Adopting a chihuahua, he instructs Steve to take the dog with him to a coffee shop each day at the same time. As Steve settles into a routine, Bishop escalates training, taking him to observe a contract killing.
The target is a mechanic for another agency named Burke, who frequents the same coffee shop. Burke’s only weaknesses are that he is interested in young men and small dogs. Burke makes his move on Steve and invites him out for drinks. Bishop instructs Steve to slip a large dose of Rohypnol into Burke’s drink to cause an overdose. Steve ignores the instructions and goes with Burke to his apartment. When Burke begins to undress, Steve attempts to strangle him with a belt as he had seen Bishop do on a former assignment. Burke fights back, using his size advantage and experience, but Steve manages to kill Burke after much effort and a lengthy fight. Dean expresses his disapproval of Bishop involving Steve, and that violated the rules of the contract arrangement, but Bishop says that Harry and his arrangement was for price only, and Harry left the details up to Bishop.
Bishop’s next contract is to kill cult leader Andrew Vaughn. They plan to inject their victim with adrenaline to simulate a heart attack, for which the paramedics would administer a fatal dose of epinephrine. While Bishop and Steve are preparing for the hit, Vaughn’s doctor arrives, and he sets Vaughn up with an IV of ketamine, which will inhibit the epinephrine’s effects. The hit men decide to suffocate him instead, but are discovered after killing Vaughn and are forced into a shootout with the guards; they slip away and fly home separately.