Shot in Puerto Rico. American secret agent XK150, using the alias Sparks Moran (Robert Towne, credited as Edward Wain), has infiltrated a gang which consists of Boss Capeto's brazenly felonious blond girlfriend Mary-Belle Monahan (Betsy Jones-Moreland); her deceptively clean-cut younger brother Happy Jack (Robert Bean); and a gullible, good-natured, and homicidal oaf named Pete Peterson Jr. (Beach Dickerson), who constantly does animal impressions.

Unfortunately, despite his other role as the story's omniscient narrator, Sparks is too much the Maxwell Smart-style bumbler to figure out what is going on because of his own incompetence and his hopeless infatuation with the completely uninterested Mary-Belle, who regards his attempts to rescue her from a life of crime with an amused contempt.