A professional assassin, portrayed by Michael Fassbender, is on a meticulous stakeout in a Paris hotel room, preparing for his mission to eliminate a target checking in at an undisclosed time. Over the course of five endless days, he fills the time with mundane activities such as eating, practicing yoga, and listening to The Smiths. He maintains communication with his handler, the lawyer and former university professor, Edward "Eddie" Hodges. Despite his desire to abandon the operation due to waiting fatigue, Hodges insists they won’t be compensated unless the job is completed.
As the assassin remains dedicated, demonstrating remarkable patience and planning, he repeatedly checks for the target throughout the night. However, during the sixth night, chaos ensues when the target makes an appearance, accompanied by a dominatrix played by Monique Ganderton. In a tragic turn of events, he misses his target and accidentally injures the dominatrix instead. Now a fugitive, he escapes on an e-scooter, evades authorities, and disposes of evidence before boarding a flight to the United States using false identities.
Communicating with Hodges about his failure, he learns that his chance to eliminate the target is indefinitely lost. Upon returning to his hideout in the Dominican Republic, he discovers his sanctuary has been violated, and his girlfriend, Magdala, is in critical condition after being attacked. With her brother Marcus by her bedside in the ICU, he reveals the horrific details of her ordeal at the hands of two assassins, who showed no attempt to mask their identities.
Determined to find those responsible, the Killer tracks down a taxi driver, Leo, who recalls transporting the assassins. He identifies one of them as a formidable man known as "The Brute" (Sala Baker) and a woman who "resembles a Q-Tip" (Tilda Swinton). Consumed by rage, the assassin breaks his foundational rule and executes Leo, taking matters into his own hands.
His journey leads him to New Orleans, disguised as a maintenance worker at Hodges' office, where he forcefully gathers information about the hitmen. Confronted by the reality of Hodges’ involvement and potential retributions, the Killer resorts to violence with a nail gun but faces an unexpected turn as Hodges does not divulge the information. A desperate Dolores (Kerry O'Malley), Hodges' secretary, promises to provide the names for her own safety, leading to a chilling moment where the Killer abandons his empathy.
After disposing of Hodges, the Killer tracks down "The Brute," culminating in a fierce confrontation that ends with fire and vengeance. His quest ultimately brings him to Beacon, New York, where he encounters the Expert, the second assassin. During an intense discussion over drinks, she tries to dismiss the personal nature of the hit, but ultimately meets her end outside as the Killer defends his motives.
Continuing his pursuit, he finds the client behind the hit, Henderson "Clay" Claybourne, and uncovers the betrayal that led to the failed assassination in Paris. Confronting Claybourne in a tense showdown, the Killer learns that this venture into contract killing was initiated at the behest of Hodges. Ultimately, he spares Claybourne's life but leaves a haunting warning: if retribution is sought, a "slow death" awaits him.
The film concludes with the Killer finding his way back to the Dominican Republic, returning to the side of a recovering Magdala, exploring themes of loyalty, vengeance, and the costly nature of his profession.
