After escaping a top-secret government facility, Nyle Maxwell (Winter) has finally reunited with his long-lost girlfriend, who he rescued from the government. Unfortunately for him, however, his girlfriend doesn't remember him or know how she even got into Capes Online in the first place.

Additionally, Winter and his team now find themselves in the Underworld, a hidden world located far below the surface of Adventure City. The Underworld is dominated by the vicious and deadly Dwellers, a race of lizard people with an innate hostility toward all humans, Hero and Villain alike. With the recent theft of their queen's precious crown at the hands of a human thief, the Dwellers are threatening to invade the surface and drag all of humanity into a devastating war no one will win.

Now Winter and his teammates must brave the dangers of the Underworld, retrieve the Dweller Queen's crown, and return it to her in three days, or else the Dwellers will invade the surface and destroy humanity. But there are far worse dangers than the Dwellers hidden in the darkest shadows of the Underworld, dangers that even Winter might be powerless to stop.