''Dr. Jekyll and Mr. Mouse'' is a 1947 one-reel animated cartoon and is the 30th Tom and Jerry short. The cartoon was released on 14 June 1947, and was directed by William Hanna and Joseph Barbera, produced by Fred Quimby and animated by Ed Barge, Michael Lah, Kenneth Muse and Al Grandmain. The episode is a parody of Dr. Jekyll and Mr. Hyde. It was nominated for the Academy Award for Best Short Subject: Cartoons in 1947, but lost to Warner Bros. Tweetie Pie.
Near a basement window are two milk bottles. Through the basement window, Tom reaches for one of the bottles and then snatches it, rushing into the living room to pour it in his bowl and drink it but Jerry also wants Tom's milk. Tom makes various attempts to catch Jerry but when they all fail, the cat dreams up a murderous plot to rid himself of his annoyance.
The next scene shows Tom mixing chemicals into his milk, upon mixing the deadly elixir, the chemicals cause the spoon to dissolve. After seeing a fly taking a little sip from the bowl before crying in pain and then dropping dead, Tom carefully carries the bowl and places it outside Jerry's hole, confident that his potion works, gleefully watching Jerry about to drink from the bowl.
Seeing the bowl of milk once again, the mouse drinks the milk, causing him to yell in pain and collapse "dead". Just before Tom can celebrate over the "dead" mouse, the full effects of the potion take place: Jerry then turns into a much bigger and stronger version of himself who is now more than able to defend himself against Tom. Tom panics and shuts the door to the living room, but Jerry knocks it over. Jerry's seemingly infinite strength allows him to smack Tom against a safe, but the potion's effects wear off and Jerry returns to his normal size.
After realizing he can no longer pull the cat around, he dives away from the cat and into the milk bowl. Tom grabs the mouse out of the bowl as he screams and turns Jerry bigger and stronger again, whom now grabs Tom and hurls him by his whiskers before the potion wears off. As Jerry realizes he is normal again he simply ties Tom's whiskers and tries to drink more potion. The feline upends the bowl with his foot before the mouse can drink enough, and the chase continues across the kitchen.
Once Tom launches Jerry into the kitchen refrigerator, Jerry hurriedly attempts to make more of the concoction himself, with a different formula but before Jerry has a chance to drink it, Tom escapes from the refrigerator and steals the milk from Jerry. The mouse tries to run away, but Tom stomps on his tail and forces his enemy to watch as he drinks the potion. The cat grins evilly at Jerry and as Tom grows to an enormous size. But then an explosion erupts from Tom, resulting him shrinking to a smaller size than Jerry.
Jerry grabs his tail from Tom's foot, grabs Tom's tail, and punches his face. This triggers the cat to shrink further, and Jerry snaps Tom's tail, Tom then shrinks down to the size of a fly, and Jerry decides to chase Tom with a flyswatter as the cartoon closes.