You are a hero raised from the dead by the god Zeus to rescue his daughter Athena. With scenery inspired by Ancient Greece, you have to fight hordes of undead and demons, with a boss at the end of each level until you meet the god Neff, who holds the girl captive.
A Roman centurion who died in battle is resurrected 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 collect three spirit balls on each level, the last of which transforms him into a human/beast hybrid of formidable power.
After a series of battles in a journey that ends in Dis, the centurion finally defeats Neff and rescues Athena. In the original arcade game, 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.
The Altered Beast title refers to your shapeshifting abilities. In this platformer, you can collect spirit balls by defeating two-headed wolves, allowing you to mutate into different beasts such as a werewolf, a dragon, a tiger, a bear and others depending on the version. Each form, tied to a level, comes with special abilities such as flight and powerful attacks, easily superseding the basic set of punches and kicks in your human form.
The game can be played in single player mode, or in same-screen multiplayer co-op mode.