While on a space mission, astronaut Tom Trimble (Dennis Dugan) and his android clone, Hermes (also Dugan), are accidentally sent back in time to the era of King Arthur (Kenneth More). Befriending the legendary English leader, Tom and Hermes must stop a scheme by the wizard Merlin (Ron Moody) and the sinister Sir Mordred (Jim Dale), who plan to dethrone the king. The duo passes off 20th-century technology as sorcery to in order to help defeat Merlin and Mordred.