The friendship of two working stiff railroad engineers is put to the test when one falls for the other’s wife.

Captain Caveman is the titular hero of the cartoon series of the same name, he is a caveman brought back to life in the modern age who fights crime and injustice with the aid of his band of mainly female friends: he is armed with a large club and is famous for his battle-cry of "CAPTAIN CAVEMAN!".

Two escaped convicts are found hiding in a rocket ship built by a renegade inventor, who forces them to become the crew for a trip to the Moon. Also on board, as inadvertent stowaways, are his assistant and his secretary; and none of them are aware that the inventor is actually a Lunarian explorer sent to Earth by the dying Lunar civilization.