During the Allies' occupation of Paris late in World War II, bumbling tank corps Sergeant Brian Ian "Boysie" Oakes stumbles when answering a fellow Englishman's call for help, and inadvertently shoots and kills two men who are attempting to assassinate British Intelligence Major Mostyn. A grateful Mostyn mistakenly believes Oakes is a born killer.
Twenty-one years later, Mostyn — now a colonel in British Intelligence — and his boss face trouble from a string of embarrassing security leaks. To save his job, the chief orders Mostyn to hire an assassin to implicitly eliminate security problems without official authorization. Mostyn recruits Oakes into MI5 without revealing the full scope of the job, tempting him with a plush apartment and a gleaming sports car. After Oakes passes a training course, Mostyn reveals that his code name is "L," short for "Liquidator," and that he is employed as a paid assassin. Unable to resign and not at all a killer by nature, Oakes secretly hires freelance contract killer Eric Sykes to handle the dirty work.
Oakes is drawn to Mostyn's secretary, Iris MacIntosh, despite a strict Agency rule against fraternization between spies and civilian staff. Determined to take a real risk, the womanizing Oakes persuades Iris to spend a weekend with him on the Côte d'Azur. But the trip goes awry when he is captured by enemy agents led by Akim Tamiroff as Sheriek, who believes he is on a mission in the south of France rather than simply on an assignation. Sheriek's superior, John Le Mesurier as Chekhov, is furious that the operation has been blown open, and he has Sheriek arrange for Oakes to escape.
Agent Quadrant, an Agency higher-up, arrives with a new mission: to stage a fake assassination attempt on the Duke of Edinburgh when he visits a Royal Air Force base, as a test of its security. Just as Oakes is about to fire a telescopic rifle loaded with blanks, he realizes that Mostyn has been maneuvered into taking the Duke's place in the troop inspection, and Quadrant is actually an enemy agent. While Mostyn and Oakes wrestle with loyalty, Quadrant and an accomplice steal an advanced Vulture supersonic nuclear bomber. Oakes manages to wound Quadrant and board the plane before it can depart. To his surprise, the pilot is Iris, who reveals she is the coordinator of the entire operation. He overpowers her, and with the autopilot engaged and an experienced Vulture pilot quietly guiding him down, he returns the aircraft to the base.
No sooner is he grounded than Oakes sets his sights on Mostyn's new — and equally captivating — secretary. Yet another wave of trouble lurks just beyond the horizon, leaving this reluctant killer to navigate a web of loyalties, deception, and dangerous assignments, all while trying to maintain his own fragile sense of right and wrong.
