The Turtles discover for the first time that they are not the weirdest things in New York when they encounter two villains from the Hidden City pursing a cat-like agent. They have their first encounter with Baron Draxum who is making mutants with the help of his Oozesquitos.

Splinter has come down with the rat flu, and the Turtles must deal with its 7 different stages involving fever, "Wild Rat Man", "Captain Cuddle Cakes", "Ninja Supreme", karaoke love songs, and fan fiction. The Turtles hope that Splinter will make to the final stage called "Must Say Yes" so that they can ask him for whatever they want.

To prove himself as the greatest enemy of the Turtles, former news anchor-turned-mutant earthworm Warren Stone gets in the way of the Turtles as they track down Hypno-Potamus, who is abducting the animal partners of other magicians.

After the Turtles stumble across a pizza restaurant in the mystical Undercity while following a Fire Yōkai, Leonardo drags his brothers into a pizza-shaped Maze of Death to obtain the ultimate prize....a free helping to the titular parlor's "world's greatest pizza". But in order to claim it, they will have to contend with the pizza's guardian in the form of the Minotaur who is the maze's assistant manager.