Written by Charles B. Griffith, the movie is a parody of spy, gangster, and monster movies (mostly Creature from the Black Lagoon), concerning a secret agent, XK150 (played by Robert Towne credited as Edward Wain), who uses the name "Sparks Moran" in order to infiltrate a criminal gang commanded by Renzo Capetto (Antony Carbone), who is trying to transport an exiled Cuban general with an entourage and a large portion of the Cuban treasury out of Cuba.

During the Cuban Revolution, deported American gambler and racketeer Renzo Capetto devises a get-rich-quick scheme and uses his yacht to help a group of loyalists commanded by General Tostada to escape with part of Cuba's national treasury, which they plan to use to stage a counter-revolution.

The gang consists of Capeto's brazenly felonious blonde girlfriend Mary-Belle Monahan; her deceptively clean-cut younger brother Happy Jack; and a gullible, good-natured, and homicidal oaf named Pete Peterson Jr., who does frequent animal impressions.

Unfortunately, despite his other role as the story's omniscient narrator, Sparks fails to realize what is happening because of his own incompetence and infatuation with the completely uninterested Mary-Belle, who regards his attempts to rescue her from a life of crime with amused contempt.

Capetto plans to steal the gold fortune and then to claim that the mythical "Creature from the Haunted Sea" rose and devoured the loyalists, but it is he and his crew who murder one of the Cuban soldiers with sharpened, claw-like gardening tools and leave behind fake "footprints" made with a toilet plunger and a mixture of olive oil and green ink.

When the monster's insatiable hunger interferes with his scheme, Capetto decides to sink his boat into 30 feet of water off the shore of a small island and then to retrieve the gold later.

Complications ensue when two of the men of his gang get involved romantically with native women, with Pete debeloping a relationship with Porcina and Jack with her pretty daughter Mango, and local prostitute Carmelita becoming fond of Sparks.

It was the last of an unofficial trilogy of black comedies by Griffith and Corman which also included A Bucket of Blood (1959) and The Little Shop of Horrors (1960).

In 1959, after the completion of The Little Shop of Horrors, Roger Corman assembled a small cast and crew and arrived in Puerto Rico to direct Last Woman on Earth and produce a World War II movie titled Battle of Blood Island.

"Everything was the same," said Griffith, "The crooks were waiting to get away in an airplane, the gangster’s moll falls for the rented hero and there’s the speech about security; everything was the same.

"[7] He felt he was not being paid enough for the job and decided to punish Corman by writing him a part of Happy Jack Monahan, the most difficult role he could think of, requiring the character to be laughing hysterically in one scene and crying like a baby in the next.

[5][8] "This fellow was a tropical Hamlet," recalls Corman, " crying one moment, laughing the next, going from killer to victim—the works.

Corman told Bean that if he could pay his own way to Puerto Rico, he could be part of the crew and play a role in one of the movies.

That's actually not a bad suggestion, considering it's got my favorite ending of them all — a last scene I invented on a whim and literally phoned to Chuck Griffith from Puerto Rico.

The final shot in this picture," I insisted, "is the monster sitting on the chest of gold at the bottom of the ocean floor.

We were shooting in a palm grove, and I had these Americans playing touch football with a coconut[...]If nothing else, there was a lot of movement in that scene.

"[4] During a scene in which the character Renzo Capetto is shown assembling an automatic pistol, actor Antony Carbone was given a real gun which fell apart unexpectedly.

[6] Actress Betsy Jones-Moreland stated of the production that "the only problem with that movie is that it started out to be a takeoff on everything Roger had ever done before.

"[16] In March 1963, Corman reassembled the cast in Santa Monica to appear in new scenes directed by Monte Hellman for television showings of the movie, expanding the length to 74 minutes.

[17] Hellman also filmed footage for television versions of Beast From Haunted Cave, Ski-Troop Attack, and The Last Woman On Earth.

According to Hellman, "That was probably the most fun I've ever had because I was the producer, writer, and director, and I had absolute control over the crew and how the money was spent and everything.