Recap: Teenage Mutant Ninja Turtles 2003 S 4 E 23 Adventures In Turtle Sitting
Donatello is sick so he is staying with Casey and April. However, he started turning into one of the mutants that the Turtles will have to fight against.
This episode provides examples of
Back from the Dead: Baxter Stockman, thanks to Bishop saving his brain and attaching it to a new cyborg body
Brainwashed and Crazy: while Donny's not quite brainwashed, he is mutated and certainly crazy enough to live up to the spirit of the trope — unwitting Face-Heel Turn included.
Call Back: All the way back to "Outbreak", which is where Don picked up the mutagen in the first place. Bishop bringing Stockman back in a new cyborg body is also a reference to "Insane in the Membrane"
Chekhov's Gun: The tunnel to April's apartment: introduced in "The Trouble with Augie"
Continuity Nod: this episode features April's basement apartment, which hadn't been seen or referenced since "The King," three seasons earlier. It even had the appropriate pieces of furniture in it, like Kirby's drawing desk.
Mikey: Now I know how a pinata feels, and I don't like it!
Resist The Beast: Donny's transformation into mindless beast is obviously painful, yet he's able to resist it for long enough to beg April and Casey to 'stay away from' him as he runs from the room, only to turn into a monster and start attacking them.