The planet Krull faces an invasion by a cosmic being called the Beast, accompanied by his formidable army of Slayers. These forces journey across the galaxy in a massive, mountain-like spaceship known as the Black Fortress. To unite their kingdoms against this threat, Prince Colwyn and Princess Lyssa arrange to marry, intending to pool their resources to combat the Beast's forces. Yet, before their union can be sealed, the Slayers launch a brutal assault, leaving the Krull armies decimated, Colwyn injured, and Lyssa taken captive.