Soul of Evil is the thirteenth and final episode of Season 4 of Super Robot Monkey Team Hyper Force Go!, as well as the fifty-second and final episode of the series as a whole. It ends on a cliffhanger, leaving the show's final conflict unresolved due to its cancellation.
The episode revolves around Sparx's corruption and betrayal of the Hyperforce, and the team's desperate efforts to stop the resurrection of the Skeleton King and rescue Sparx from his corruption.
Sparx, now corrupted by the Fire of Hate from the prior episode, abandons the Monkey Team, and he, Valeena and Mandarin make their escape together. In their leave, the Hyperforce are forced to content with a large Gamera-esque lava turtle, giving the villains an ample headstart.
The group return to the Super Robot to prepare for battle, with Jinmay operating the right Fist Rocket in Sparx's absence. Despite their best attempts, the monster refuses to buckle and, in a last ditch effort Gibson reverses the polarity of the Super Robot's heat rays, which freeze the monster solid as a result. They proceed to shatter the now frozen behemoth and fly off after Valeena.