In 1944 during World War II, tank corps Sergeant "Boysie" Oakes (Rod Taylor) stumbles and unwittingly shoots and kills two men attempting to assassinate British Intelligence Major Mostyn (Trevor Howard) in Paris. Mostyn mistakenly believes Oakes was lethal on purpose.
Twenty years later, Mostyn (now a colonel in British Intelligence) and his boss (Wilfred Hyde-White) are in trouble due to a series of embarrassing security disasters. To save his job, the chief orders Mostyn to hire an assassin to illegally eliminate security leaks without official authorisation. Mostyn recruits Boysie into the Secret Service without first telling him what his employment will entail, luring him in with a lavish apartment and a fancy car. After Boysie passes a training course, Mostyn informs him that his code name is "L", and that it stands for liquidator. Unable to resign and not a killer himself, Boysie secretly hires a freelance professional assassin (Eric Sykes) to do the dirty work.
Things go well until Oakes persuades Mostyn's secretary Iris (Jill St. John) to spend the weekend with him on the Côte d'Azur, though Mostyn has warned him that any contact between spies and civilian employees is a serious criminal offence. Boysie is captured by enemy agents led by Sheriek (Akim Tamiroff), who firmly believes he is on assignment and wants to know who the target is. However, Sheriek's superior, Chekhov (John Le Mesurier), is coldly furious that he has gone beyond his orders to merely watch Boysie, thus endangering a much more important operation. He has Sheriek arrange for Boysie to escape.
Then Quadrant (David Tomlinson) arrives with a new mission for Boysie. He is to stage a fake assassination attempt on the Duke of Edinburgh when he visits a Royal Air Force base to test the security. Boysie has been duped: Quadrant is actually an enemy agent, and the bullets in his sniper rifle are real. Mostyn shows up in the duke's place and is able to locate Boysie, but while they are distracted, Quadrant and a pilot steal the real target: the Vulture, an advanced new aircraft which the duke was to inspect. Boysie manages to shoot Quadrant and board the plane as it is taking off. To his surprise, the pilot is none other than Iris, who informs him that she is the coordinator of the operation. He is able to overpower her and, with radio help, return the aircraft to the base.
Producer Jon Pennington brought Australian screenwriter Peter Yeldham to the project after both had cooperated on The Comedy Man (1963). Yeldham recalled that Pennington acquired the novel, read it on an airplane and set the film into production in four or five months. Like the Jason Love Where the Spies Are, also filmed in MGM-British Studios, MGM planned a Boysie Oakes film series. Producer Sydney Box spoke to Yeldham and wished him to write two more scripts in the projected series .
Richard Harris was initially approached for the role but after negotiations chose to do The Heroes of Telemark instead. Taylor insisted on playing the role with an American accent because he was more comfortable with it by that stage in his career.
The film opens with animated titles by the Richard Williams studio.
Like Where the Spies Are, was also filmed in MGM-British Studios . Cardiff recalled that the censors made them delete one of Taylor's lines: "it smells like a Turkish wrestler's jockstrap".
Release of the film was held up a number of months due to a legal conflict between producer Leslie Elliot and MGM. Jack Cariff thought this hurt the final box office result of the film, which was disappointing.
The original score was composed by Lalo Schifrin and includes a driving main title vocal theme and a soft end title theme ("My Liquidator"), both sung by Shirley Bassey. Other than the "Goldfinger"-type title song, Lalo Schifrin deliberately avoided the John Barry James Bond style of music.
The Liquidator was released to DVD by Warner Home Video on 6 September 2012 via the Warner Archive DVD-on-demand service.