Plot

Solitary, cold, methodical and unencumbered by scruples or regrets, a killer waits in the shadows, watching for his next target. Yet, the longer he waits, the more he thinks he's losing his mind, if not his cool.

A man solitary and cold, methodical and unencumbered by scruples or regrets, the killer waits in the shadows, watching for his next target. And yet the longer he waits, the more he thinks he's losing his mind, if not his cool. A brutal, bloody and stylish noir story of a professional assassin lost in a world without a moral compass, this is a case study of a man alone, armed to the teeth and slowly losing his mind.

A professional assassin (Michael Fassbender) stakes out a Parisian hotel room. He prepares to use a sniper rifle to kill a target who will check into the hotel room at an unspecified time. While waiting for the target, he eats, practices yoga, listens to The Smiths, and talks on the phone with his handler, an attorney, and his former university law professor, Edward "Eddie" Hodges (Charles Parnell). The assassin has been waiting for 5 days for the target to arrive, and wants to leave, but Hodge insists that they don't get paid, unless the job is carried out. The assassin is shown to be patient and methodical. He builds redundancies into his plans. He likes to observe the cities in which he operates and observes the people in these cities and their daily routines. His identity while he is on the road is of a German tourist. But for 5 nights, the assassin has been waking up every hour to check if the target has arrived. He is sleep deprived and weary. Then on the 6th night, there is action in the middle of the night.

The target arrives with a dominatrix and the Killer misses his target, accidentally shooting the dominatrix (Monique Ganderton). He flees on an e-scooter, evading the police and disposing of his rifle and gear. He then flies to the United States under fake identities such as Felix Unger, Archibald Bunker. The killer informs Hodges about his failure. The window of opportunity to take out the target is closed now for a long period of time.

The Killer returns to his home and hideout in the Dominican Republic to find it has been broken into and his girlfriend Magdala (Sophie Charlotte) has been attacked. He finds boot prints all around the hideout to indicate that the attackers staked the house for a while before making their move. She is in an intensive care unit of a hospital, with her brother Marcus (Emiliano Pernía) watching over her. He says that Magdala was interrogated and tortured by two assassins (one of which was a woman) but managed to injure one of them and escape. The killers were not interested in disguising themselves. Marcus says that after they were done, Magdala saw them leave in a car with a green top, like a taxi. The Killer tracks down the taxi driver (there is only one company that runs a green color taxi in Dominican Republic), Leo (Gabriel Polanco), who drove the assassins to the Killer's home. Leo identifies one of the assassins, a strong man with an injured leg; "The Brute" (Sala Baker), and a woman who "resembled a Q-Tip" (Tilda Swinton). Leo picked them up from a private airport, drove them according to their instructions, waited for them an hour at the Killer's property and then drove them back to the airport. The Killer kills Leo, and he breaks his "Fight only the battle you're paid to fight" rule and seeks to track down the two assassins.

The Killer travels to Hodges' office in New Orleans, Louisiana, and enters the building disguised as a custodial worker. It is revealed that Hodges used to teach law to the Killer, but then encouraged to venture out into contract killing. After forcing Dolores (Kerry O'Malley), Hodges' secretary, to restrain her boss and herself, the Killer destroys the records of their work. Hodges says that he had to make retributions to a very angry and powerful man, and he never thought that the Killer would return home. As Hodges attempts to get the Killer to leave, the Killer shoots him in the chest with a nail gun to torture the names of the hit-men out of him. However, Hodges doesn't reveal the names and dies quicker than the Killer anticipated. Dolores offers to show the Killer the assassins' identities in her paper files in her home, asking in return that the Killer give her a non-suspicious death so her children can claim a life insurance payout. After receiving the names of the 2 assassins and the client who paid Hodges, at her home, the Killer breaks his rule not to show empathy; he snaps her neck and shoves her down a flight of stairs, making the fall look like an accident. He later disposes of Hodges' body on a ship.

Afterward, the Killer drives to St. Petersburg, Florida, identifying the Brute by his limp. The Killer tracks the Brute through the day as he travels to a casino with his friends and returns late at night. After drugging his pit bull, the Killer enters his house to kill him, only to be blindsided and tackled. They engage in a fight, and the Brute recognizes the Killer. The Killer shoots him and runs from his reawakened dog, burning down his home with a Molotov cocktail.

The Killer travels to Beacon, New York. He tracks the 2nd assassin from her home as she leaves in her car and confronts The expert "Q-Tip" assassin in a gourmet restaurant. Appearing to accept her fate, the Expert questions the Killer's motivations in continuing and lessening competency in his profession, as they partake in her "last supper" of whiskey. The Expert tries to argue that it was an address and a target and nothing personal and that she had nothing to do with what Brute did to Magdala. In a park outside, the Expert appears to trip on the stairs and requests the Killer to give her a hand. The Killer, instead, kills her and notices she had a concealed knife in her hand.

The Killer travels to Chicago, where the client (Arliss Howard), billionaire venture capitalist Henderson "Clay" Claybourne, lives in an upscale penthouse. The Killer transfers all of his finances to the Dominican Republic under the name Jefferson and observes Claybourne's routine. The Killer then uses tools purchased online to clone Claybourne's key-card and enter his penthouse. The Killer confronts Claybourne at gunpoint in his penthouse, asking him why he ordered the retaliatory hit. Claybourne claims that he has no personal problem with him. After the miss in Paris, Claybourne claims that Hodges called him and informed him of the miss. Claybourne was OK as mistakes do happen, and it was Hodges who sold him on the idea of a cleanup to prevent any blow-back from his target. As a first-time client of a hit-man, agreed to pay Hodges for "the trail to be scrubbed" for an additional $150,000. The Killer spares Claybourne, though promising him a "slow death" if Claybourne ever comes for him. Later, The Killer returns to the Dominican Republic and sits next to a recovering Magdala.