The Ghost Detective Club has a new case. The client is a professor that uses Deco overlays to create artificial animal that he has on display at his Neo Animal Museum. His self-proclaimed masterpiece, the Nue, has an appearance that doesn't match the listing on Decopeedia, and hires the detective club hoping to clear up the confusion. After getting a glimpse at the professor's Nue, it looks nothing like the cute creature whose picture is on the Decopedia page. Even so, the professor insists that his Nue is the accurate version.

When Finn was young, his family lived in a slum where garbage tended to be dumped. One day, his brother brings him an older model of Deco that should be easier for Finn to use due to his Deco-Sickness. With it, Finn accidentally discovers that the local purification systems are broken. Believing this is what has caused his home to become polluted, he uses the Deco that he got from his brother to request that the customer center fix the purification system. But because he isn't registered as a citizen, his login is flagged as illegal, which leads to his request not being answered.

Thanks to Doron and his drone friends, Berry, Hack, and Finn are able to reach Mark Twain, a strange facility floating in the sky. By using the Angelfish club card they had previously obtained, they manage to get inside, where Berry and Hack enter Mark Twain's rendition of the Hyperverse. What they discover appears to be a labyrinth. Knowing that all the secrets of Tom Sawyer island must be contained within, Berry and Hack start searching for Phantom Zero's identity.