Grendel (voiced by Ustinov) is by turns a thoughtful and contemplative character and a rampaging monster who attacks the mead hall of an early Danish kingdom, biting the head off of one would-be defender.

This was the second full-length fully animated film ever made in Australia (coming after 1972's Marco Polo Junior Versus the Red Dragon).

Initially narrated by the titular character through a flashback, Grendel (Peter Ustinov), the "Great Boogey", recounts how he first left his cave as a child and encountered the Danish King Hrothgar (Ed Rosser) and his thegns.

He watched as Hrothgar's power and wealth grew, disgusted at his excesses and the royal Shaper's (Keith Michell) revision of history, presenting the king's underhanded and brutal achievements as glorious victories.

Before Grendel can arrive, the meadhall is visited by the hero Beowulf (also voiced by Dignam), who kills Unferth on the increasingly paranoid Hrothgar's orders.