The movie's based on John Gardner's novel of the same name. I'd read the book and knew pretty much what to expect. Rod Taylor plays Boysie Oakes, a reluctant spy whose job is assassination. There were probably many instances before this of similar situations (most often in Bob Hope movies). The main character is hopelessly inept, but because of mistaken impressions of him by all concerned, he appears to be quite heroic and dangerous, a regular killing machine. Mainly he's a coward who's very lucky. One good twist (not really a spoiler) is that Boysie likes being The Liquidator, but being unable to do the job, he hires a professional hit man to take care of the unpleasantness.
So you know what you're getting here. You also get Jill St. John in Technicolor, along with Trevor Howard and Akim Tamiroff, among others. Plus good production values and Shirley Bassey singing the title song. Good (but not great) fun.