The planet Krull is invaded by an entity known as “The Beast” and his army of futuristic “Slayers”, who travel the galaxy in a mountain-like spaceship called the Black Fortress.
The Slayers attack the wedding of Prince Colwyn (Ken Marshall) and Princess Lyssa (Lysette Anthony) – who are marrying in a bid to form an alliance between their rival kingdoms in the hope that their combined forces can defeat the Beast’s army – killing the two kings, devastating both armies and kidnapping the princess.
Prince Colwyn is found and nursed by Ynyr, the Old One (Freddie Jones). Ynyr tells him the Beast can be defeated with the “Glaive”, an ancient, magical, five-pointed throwing weapon.
Colwyn sets on his quest and retrieves the Glaive from a high mountain cave before setting out to track down the Black Fortress, which teleports to a new location every day at sunrise. As they travel, Colwyn and Ynyr are joined by magician Ergo “the Magnificent” (David Battley).
Colwyn also rounds up further reinforcements of criminals, promising to clear their criminal records if they help, including Torquil (Alun Armstrong), Kegan (Liam Neeson ), Rhun (Robbie Coltraine), Oswyn (Todd Carty), Bardolph (Dicken Ashworth), Menno (Bill Weston), Darro (Andy Bradford), Nennog (Bronco McLoughlin), Quain (Gerard Naprous) and Rell the Cyclops (Bernard Bresslaw).
Once assembled they set off to destroy The Beast and rescue the Princess.