Last Woman on Earth tells the story of three survivors of a mysterious apocalypse, which appears to have wiped out all human life on earth.

Harold Gern (Antony Carbone), a successful businessman from New York who is constantly in legal trouble, is spending a holiday in Puerto Rico with his attractive wife, Evelyn (Betsy Jones-Moreland), whom he married "between trials." They are joined by Martin Joyce (Robert Towne, billed as Edward Wain), Harold's lawyer, who has come to discuss the latest indictment.

Harold invites him along on a boat trip during which all three try out some newly bought scuba diving equipment. When they resurface, they are unable to breathe without using their scuba tanks. They climb back into their boat and find Manuel, the crewman, dead, apparently of asphyxiation. Upon rowing ashore, they enter the jungle. With their air running out, they discover that the foliage gives off oxygen that they can breathe.

When they go into town, they find nobody left alive and cannot contact anyone by radio. It dawns upon the three that they might be the only survivors in the area, maybe in the world. The domineering Harold takes charge. They becoming self-sufficient, and the two men fish, as marine creatures have survived. Later, they also find living insects and baby chicks, presumably newly hatched. Harold feels that in the long run, they will have to move north to a colder climate to avoid problems with insects and food preservation and to increase their chances of meeting other survivors.

Soon, a love triangle develops. Martin points out to Harold that neither the latter's marriage certificate nor his money mean anything anymore. One day, while Harold is out fishing by himself, Evelyn gives in to her attraction to Martin. When Harold finds out, he beats Martin and orders him to leave. Evelyn hops in the car, and the two lovers drive off. Harold chases after them. At the harbor, another fight breaks out when Martin refuses to tell Harold where his wife is. Martin eventually runs into the church where Evelyn has been waiting. There, he dies of his injuries. The two survivors are left wondering where they will go or what they will do now.

Directed by Roger Corman
Starring
Betsy Jones-Moreland
Antony Carbone
Robert Towne

text from Wikipedia