A Roman centurion who had died in battle is resurrected from the dead by Zeus. The centurion is ordered by Zeus to save his daughter Athena from a Demon God called Neff in the Underworld. To become able to withstand the perils, the warrior gets the ability to absorb spirit balls which transform him into an Altered Beast, a part animal, part human creature of formidable force. Between each level are small animations giving the player glimpses of Athena's peril. He first travels through the graveyard fighting past zombies and winged demons. The centurion becomes a werewolf in order to fight Neff in the form of Aggar. The centurion defeats him, but Neff escapes.
The centurion travels through the Underworld where he fights through giant leeches, Chinese dragon-headed rattlesnakes, and ferocious Chicken Stingers (resembling the ones from Golden Axe). The centurion turns into a humanoid thunder dragon in order to fight Neff in the form of Octeyes. The centurion defeats him, but Neff escapes.
The centurion travels through the Cavern of Souls where he fights through Cave Needles (which resemble giant ants), turtle/snail hybrids, and zombies known as Grave Masters. The centurion turns into a humanoid bear in order to fight Neff in the form of Mouldy Snail. The centurion defeats him, but Neff escapes.
The centurion arrives at Neff's palace where he fights through zombies, Chicken Stingers, and Hammer Demons. The centurion turns into a humanoid tiger in order to fight Neff in the form of Crocodile Worm. The centurion defeats him, but Neff escapes.
After the centurion arrives in Dis, he fights through winged demons, Goat Men, Boar Men, saw fishes, and purple Unicorn Men to get to Neff. When the centurion confronts Neff, the centurion becomes a golden werewolf while Neff transforms into a Rhinoceros-Man. After he is defeated, a blue bird comes out of the ground and transforms back to Athena. Afterwards she is shown holding arms with her lycanthropic hero, thanking him for rescuing her.
In the original arcade, the end credits are interspersed with images of actors in costumes for the different characters and monsters of the game, implying the whole game was a film production.