It is the future (well 1980 anyway) and USAF spaceships are going missing, thrown off course by a mysterious force. Captain Chapman (Dean Fredericks) is sent to investigate, his ship is also seized by a force and his ship lands on an unknown planet. Chapman heads out in his spacesuit and sees tiny people, then he passes out and his suit is opened. When he breathes the air of this planet he is also shrunk! Luckily for the film's classification although the rest of his clothes remain normal size his underpants also shrink with him...
Chapman is captured by the people of this planet, he is told he cannot leave but can have his choice of two beautiful women (Coleen Grey and Dolores Faith) as a wife! That doesn't go down well with one of the girl's beau Herron (Anthony Dexter) who challenges Chapman to a deadly duel. When Chapman wins but spares Herron's life they become friends. Herron offers Chapman the chance to return home but first he must help them battle the brutal aliens Solarites who are attacking...
So fairly familiar stuff, the plot is mostly what you may have seen before in other films (and probably made better too). The shrinking of Chapman is an interesting twist with some decent effects. The film is not great but is perfectly reasonable.
