Book Three of the Masks series follows Eric's adventures as a newly-transformed supervillain sidekick. Taking advantage of Eric's relationship with Peter, the Devil's Trill uses him for a shield against the superheroes. In the meantime, new villains and a new, covert vigilante-like group appear, with a young hero with chameleon powers attempting to infiltrate the Trill's hideout and help Eric.
Eric struggles with his conscience and schemes to turn the tables on the Trill, but his powers deteriorate. He grows more and more unstable and unsafe while the Trill's henchmen appear to grow stronger and stronger, as though they were also subjected to the same manipulation that's been used on Eric. As the Trill fights both the heroes and tries to assert his dominance over the new villains, Eric realizes that he doesn't have much time left to set things right on his own, even if it costs him his life.