Victor Maynard is a middle-aged, solitary assassin, who lives to please his formidable mother, despite his own peerless reputation for lethal efficiency. His professional routine is interrupted when he finds himself drawn to one of his intended victims, Rose. He spares her life, unexpectedly acquiring in the process a young apprentice, Tony. Believing Victor to be a private detective, his two new companions tag along, while he attempts to thwart the murderous attentions of his unhappy client.
Victor Maynard (Bill Nighy) is an expert and effective assassin living a lonely life in accordance with his family's business. Victor follows a family line of professional assassins, and he completes his assignments quickly and without remorse. One afternoon, after killing one of his targets, he hesitates in killing the pet parrot, Roger, and instead takes him as a gift to his mother, Louisa (Eileen Atkins) a formidable woman who was, until recently, also Victor's housemate. In celebration of his 55th birthday, she gives him a leather bound book with newspaper clippings of each of his kills from his first to his most recent, leaving pages for future hits to be included. She also expresses concern that he might be gay, wondering why he hasn't produced a successor.Rose (Emily Blunt) is a not-so-average girl with a talent for thievery. Her most recent theft involves the sale of a fake Rembrandt painting (painted by her friend in the Restoration Department of the National Gallery) to Ferguson (Rupert Everett), managing to swindle him out of £900,000. Ferguson soon discovers the swap and hires the best hitman, Victor Maynard, to dispose of her. Victor takes the case and immediately tracks Rose down, missing several opportunities to kill her, and accidentally killing a random market stall customer in a changing room. He follows her to a balcony opposite her hotel room and tries to shoot her through the window, but is interrupted by the arrival of the front doorman. Victor sets up a microphone and headset to keep her under surveillance, but falls asleep, unable to listen to their noisy lovemaking. He wakes the following morning, just as she is leaving. He has the opportunity to shoot but hesitates.His mother, Louisa, is upset by this missed target (and has apparently murdered Roger with a knitting needle) and suggests that Victor apologize to his employer and offer to do the hit for free. He tracks Rose down in a parking garage where he sees another hitman ready to kill her. He takes the preemptive shot, killing the other assassin. He and Rose get into her car, only to be forced out again by Mike (Gregor Fisher), another assassin hiding in the back seat of her Mini. Mike knocks Victor's gun away and lines them up against the wall to be shot and killed, but instead is wounded by Tony (Rupert Grint), an apparently homeless young man who had picked up the dead man's gun. Saying it was his first time handling a firearm, he impresses Victor enough for Victor to consider making Tony his protégé, but he sends Tony home and Victor and Rose flee. Mike starts firing at them and they nearly run over Tony on his way out of the garage, forcing him to join the ride. Rose offers Victor his price of 30,000 pounds a week for her protection, believing that he is merely a private detective. They travel to a luxury hotel where they can lie low, but by chance get a room on the same floor as Ferguson. Ferguson hires Dixon (Martin Freeman), reputed to be second only to Maynard in proficiency, to kill Rose and Maynard. After several close calls, Mike, who is also Ferguson's bodyguard, discovers their whereabouts when he spots a pair of boots that Rose had stolen from his dead partner. Tony is attacked in the bathroom and nearly drowned in the bathtub by Mike, but he turns the tables and accidentally shoots Mike's ear off before the three of them escape the hotel. Ferguson and Mike pursue them in a high-speed chase through the streets of London until Mike loses control and crashes, sending the pair to the hospital.They travel to Maynard's home, a quaint farm far in the country, where his furniture is shrink-wrapped and his cat, Snowy, resides with him. Maynard takes Tony on as his apprentice in "private detective" work. One night, Rose is attacked by Louisa, who had come back to the house to finish what her son had started. He eventually talks her down and after she leaves, the three of them work on becoming friends. Rose and Tony help Victor celebrate his birthday, and, after a brief period of sexual confusion between Tony and Maynard, Victor falls in love with and sleeps with Rose. Afterward, his attitude becomes more cheerful, and Victor peels off the plastic coverings on all of his furniture and opens up the house. Meanwhile, Rose snoops around Victor's room, finding the leather book that his mother had given him and learning that she was actually his target for assassination. She also finds Victor's father's first gun, a Broomhandle Mauser, and takes it for self-protection. She storms out of the house after making it clear that she trusts neither Victor nor Tony, and returns to the National Gallery, only to find her friend dead and Dixon and his assistant, Fabian, (Geoff Bell) waiting for her. They quickly return to Victor's home, and Tony and Victor gain the upper hand when Louisa appears, killing Fabian with a machine gun. Dixon withdraws the old gun Rose had taken from Victor's room and fires at Victor. It backfires, sending the bolt into his skull. Victor, Tony and Rose bury the pair in the back yard and return to their lives.Three years later, Victor and Rose are married with a son named Angel and Tony has moved in with them. While Angel is playing one morning, Tony comes outside asking Victor and Rose where the cat had gone off to. They look at Angel in awe as he is innocently patting soft dirt into the yard, suggesting he killed and buried the cat. Victor smiles with pride.