A comedic twist on the old-fashioned Jekyll & Hyde story.
Two characters end up together inside a Mad Scientist's laboratory. They may have been sent there to investigate the place, or may have stumbled into it by chance.
Either way, during their stay, The Fool of the two stupidly drinks some concoction found lying around, or activates some Transformation Ray, or otherwise messes with a piece of phlebotinum, resulting in him being changed into a twisted Hyde-like version of himself with greenish skin and mayhem on the mind.
The slightly-less foolish character stumbles upon the monstrous form of The Fool and suffers some sort of Amusing Injuries as a result. Now, terrified that there's some kind of creature loose in the lab, he sets out to find The Fool and get the heck out of there.
Hilarity Ensues as his attempts to find safety with (and for) The Fool only put him in murderously close range to the actual threat. The Fool has no idea he is the monster, and has no ability to control when and where his transformations occur.
This trope may be related to Mistaken for Own Murderer, in which a disguised/transformed person is attacked for killing his ordinary self.
- Abbott and Costello Meet Dr. Jekyll and Mr. Hyde: During a struggle, Jekyll's serum needle is dropped into a couch cushion, which Tubby (Costello) accidentally falls onto, transforming him into a Hyde-like monster. A mad-cap chase ensues, with Bruce chasing Jekyll's monster and Slim (Abbott) pursuing Tubby's monster (both believing they are after Jekyll).
- One of the Dragonlance short stories had a Kender stumble upon (i.e. steal) a magic dragon figurine. It turns him into a dragon at various times.
- Friz Freleng really liked this trope: he directed the Looney Tunes cartoons "Hyde and Hare" (with Bugs Bunny and the real Dr. Jekyll), "Hyde and Go Tweet" (with Tweety and Sylvester), and "Dr. Jerkyl's Hide" (with Sylvester and a pair of dogs). When he later went to work on Pink Panther cartoons, the same plot was recycled in an episode of "The Inspector" ("Sicque! Sicque! Sicque!") and again with the Pink Panther himself, where the monster was Dr. Jekyll's old car he bought used ("Pink Lightning").
- The Huckleberry Hound Show: Huckleberry Hound is faced with a pastiche of Dr. Jekyll in the cartoon "Picadilly Dilly".
- Tom and Jerry did it in 1947 with "Dr. Jekyll and Mr. Mouse", and in 1975 with "See Dr. Jackal and Hyde!".
- Dexter's Laboratory had an episode such as this, in which Dee Dee was the monster.
- Mr. Meaty has an episode of this.