This is one of the few filler episodes of Season 2. It's like that because unlike previous episodes, no new characters (like Rex, Punk Frogs, or the Neutrinos) were in it, and it wasn't part of the Eye of Zarnov.
Anyhow, April wants to find out more about Shredder, who has just made a transport device. April gets crossmutated with a cat, and turns into a cat herself! So the Turtles and Irma must stop April from killing Splinter before the show has a very "cat"aclysmic ending.