The film begins with a mysterious man arriving at a private hangar as his plane touches down, escorted by armed guards. He quickly changes into his swimwear and dives into his luxurious pool. While swimming, he spots his watch at the bottom and swims down to retrieve it, but an unknown assailant seizes him, holding him underwater until he takes his last breath. The skilled assassin, Arthur Bishop, capitalizes on the ensuing confusion to discreetly exit the mansion and slip into a nearby river for his escape.
Once back at his home, Bishop meets Harry McKenna, a fatherly figure and mentor. During their conversation, Harry compensates Bishop for a recent job he conducted in Colombia, and they touch upon Harry's son, Steve McKenna. After parting ways, Bishop returns home only to discover an unexpected contract—his next target is Harry himself. Confused, he contacts his employer to confirm the order, leading to a face-to-face meeting with Dean, who reveals a botched mission in South Africa, suggesting Harry was involved in leaking crucial information. Forced into a corner, Bishop carries out the contract, staging Harry's death as a carjacking.
At Harry's funeral, Bishop encounters Steve, who vows to track down the carjacker and obtain his revenge. Recognizing Steve's potential, Bishop decides to mentor him in the art of assassination, taking him under his wing as a "Mechanic." Adopting a chihuahua, he instructs Steve to bring the pup to a café daily. The training intensifies as Bishop immerses Steve in a contract, showing him the meticulous planning involved in an assassination.
The next target is a rival mechanic named Burke, whom Steve is instructed to eliminate. However, when Burke takes a liking to Steve and invites him for drinks, he neglects Bishop's warning to drug Burke's drink and instead attempts to strangle him. A fierce struggle ensues, and Steve ultimately manages to kill Burke. Bishop's disapproval of Steve's approach brings tension between him and Dean, revealing deeper complicities.
Their next assignment involves taking down Andrew Vaughn, a cult leader. As they plot to execute Vaughn in a hotel room, the presence of a doctor administering ketamine forces them to change tactics, resulting in a suffocation. The chaos of a shootout ensues as they flee the scene, but not before discovering that Dean manipulated Bishop all along. The realization of betrayal ignites Bishop's quest for vengeance.
During a frantic search, Bishop finds that Steve has been ambushed at his home. Directing Steve to find a hidden weapon, they engage in a fight to survive. However, as Steve uncovers the truth about his father's death, he is driven by anger. In a tragic twist, he blows up Bishop's truck using Harry's gun, leading to an orchestrated detonation that also destroys Bishop's home.
As the dust settles, a security camera reveals Bishop narrowly escaping the explosion. With newfound determination, he commandeers a vehicle and drives away, leaving a trail of chaos in the wake of their deadly game.
