Victor Maynard (Bill Nighy) is a middle-aged, solitary assassin, who lives to please his formidable mother, Louisa (Dame Eileen Atkins), 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 (Emily Blunt). He spares her life, unexpectedly acquiring in the process a young apprentice, Tony (Rupert Grint). 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.