Gadget Man comes out of retirement after 50 years, but is tricked by Shredder into fighting against the Turtles.

After many failed attempts from his rodent subjects to find food, the Rat King learns of an abandoned canister of mutagen lost in the sewers and claims it for himself. Then, he kidnaps Irma and Vernon and uses the mutagen to transform them into his mutant slaves, for the sole purpose of raiding food warehouses. It is later discovered that the mutations are unstable and can only be active if the Rat King plays his hypnotic flute.