The Emperor of Space sends a ship on a mission to a dark region of space called the Haunted Stars to destroy the home planet of the evil Count Zarth Arn. As they approach the planet, the ship is attacked by "red monsters" that look suspiciously like the inside of a lava lamp.
They recruit Stella Star (played by B-Queen Caroline Munro) to locate the crashed ship, which had the Emperor's son on board. Most of the movie takes place in various cardboard cutout garage sets that are smaller than my living room, cut apart by spectacularly boring space battles that make no sense at all. The search for the crashed ship does take our heroes to a series of different locales that pass for alien planets. The first is a beach where they are immediately captured by amazons on horseback and attacked by a giant tin statue.
Next they travel to a snow planet where the temperature drops "thousands of degrees" after sundown, so Stella is forced to cover herself with spandex. The final stop is the Phantom Planet of Count Zarth, where Stella is beset upon by vicious cavemen who destroy her cowboy robot companion and capture her. She is rescued from the troglodytes by a mysterious stranger in a ridiculous helmet who blasts them with laser eye beams until they run away. He unmasks to reveal himself as David Hasselhof, and we come to learn that he is of course the Emperor's missing son.
They make their way to the secret generators of the lava lamp space monsters only to be captured by Zarth and his men. Zarth's plan is to draw the Emperor to the Phantom Planet with the promise of returning his son, then blow it up with everyone on it. Zarth leaves them to be guarded by duck-billed robots with pirate cutlasses, and a clunky awkward stop-motion matte on live action footage laser swordfight ensues.
The Emperor arrives with only a moment before the countdown concludes, and, in the movie's finest moment, commands his battleship to "halt the flow of time" to secure their escape.
After that there's an unbelievably long and repetitive space battle where they essentially fire torpedoes full of soldiers through the windows of Zarth's space fortress (his fortress in space, folks) to invade it.
The Emperor (have I mentioned he's played by Christopher Plummer?) takes a moment to render a final monologue to the camera about how evil has been temporarily defeated, and the curtain falls on what's probably the most ambitiously terrible piece of crap I've ever seen.