During the Cuban Revolution, deported American gambler and racketeer Renzo Capetto (Anthony Carbone) comes up with a get-rich-quick scheme and uses his yacht to help a group of loyalists headed by General Tostada (Edmundo Rivera Alvarez) escape with Cuba's national treasury which they plan to use to stage a counterrevolution.
American secret agent XK150, using the alias Sparks Moran (Edward Wain a.k.a. Robert Towne), has infiltrated the gang which consists of 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-naturedly 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 ever really figure out what is going on due both to 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.
Capetto plans to steal the fortune in gold and claim that the mythical "Creature from the Haunted Sea" rose up and devoured the loyalists, while in fact it is he and his crew who murder the Cuban soldiers with sharpened claw-like gardening tools and leave behind "footprints" made with a toilet plunger and a mixture of olive oil and green ink. What Capetto doesn't know however is that there really is a shaggy, pop-eyed sea monster lurking in the very waters where he plans to do the dirty deed and that the creature may make his plan all too easy to pull off!
When the monster's insatiable hunger upsets his scheme though, Capetto decides to sink his boat in 30 feet of water off the shore of a small Puerto Rican island and then retrieve the gold at a later time. Complications ensue however when the male members of his gang get romantically involved with the natives, with Pete hooking up with the aptly named Porcina (Esther Sandoval) and Jack with her pretty daughter Mango (Sonia NoemÃ­ GonzÃ¡lez), while local working girl Carmelita (Blanquita Romero) takes an instant if inexplicable liking to Sparks.
Capetto and his gang go scuba diving to attempt to salvage the loot, but the creature picks them all off one by one except for Sparks and Carmelita, and the movie ends with it sitting on the undersea treasure happily picking its teeth.
Who does Pete hook up with?
Porcina