With only Gibson and a half damaged Super Robot, Chiro must mount a rescue against the Citadel of Bone to save the Hyperforce from tortuous brainwashing by Mandarin and Sakko. As Chiro and Gibson explore the citadel's nightmarish interior they discover secrets of Chiro's past as well as a secret of Skeleton King himself. Can Chiro harness the Power Primate, free his friends, and defeat Skeleton King before he unleashes a wave of evil on Shuggazoom?

A mysterious underground cult steals the Skeleton King' skull to gain limitless power, leading to a series of curses that fall upon Shuggazoom. The Hyperforce investigates beneath the city to stop the cult and the catastrophes. Meanwhile, witch Valeena and Mandarin set out to the same location to retrieve the skull.