It is one of approximately eight billion works of fiction inspired by The Strange Case of Dr. Jekyll and Mr. Hyde. After the morning milk is delivered, Tom sneaks out and steals a bottle off the front porch. He pours it into a bowl to drink but Jerry gets a straw and starts drinking as well. After Tom's increasingly frantic attempts to stop Jerry from drinking the milk fail, he resorts to murder. Tom dumps all sorts of poisons from the closet into the milk and sets it out for Jerry to drink.
Unfortunately for Tom that decision backfires. The milk doesn't kill Jerry; instead it turns him into a super-mouse, three times bigger and super-strong. This new Mighty Mouse version of Jerry promptly turns the tables on Tom.
Dr. Jekyll and Mr. Mouse contains examples of:
- Ate the Spoon: When he poisons his milk and starts stirring it, the spoon Tom was using completely disintegrates, to his surprise. The same thing also happens later when Jerry attempts to recreate it.
- Attack the Tail: Jerry sticks Tom's tail in a waffle iron and burns it, causing it to turn into a waffle.
- Chased Off into the Sunset: Ends with Jerry chasing an ant-sized Tom around the house with a fly swatter. Bonus points given Tom's a villain, this was his defeat in this episode, and he hardly knows when to fold 'em.
- Cranial Eruption: Predictably, Tom's attempt to whack Jerry with a club when Jerry is on top of his head leads to Tom raising a lump on his own head.
- Didn't See That Coming: Type III. Tom plans to kill Jerry via having him drink poisoned milk, but the poison instead acts as a Super Serum.
- Epic Fail: During an attempt to kill Jerry, Tom poisons his milk and has him drink it. Instead, however, it turns Jerry into a super-mouse with invulnerability and Super-Strength.
- Exit, Pursued by a Bear: The ending features Jerry chasing a now-smaller Tom with a flyswatter.
- Eye Pop: Six pairs of Tom's eyes project out in front of him upon seeing Jerry still sipping his milk after he thought he locked him in a trunk.
- Hoist by His Own Petard: When he has Jerry at his mercy, Tom drinks his attempt at replicating his Super Serum. Instead of giving him superpowers, it causes him to shrink past Jerry's height and for him to defeat him.
- The Hunter Becomes the Hunted: This happens every time Jerry turns into the super-mouse and chases Tom. And again at the end, after Tom's drinking the poisoned milk causes him to greatly shrink.
- Impact Silhouette: Tom's attempts to whack Jerry with the fireplace poker do nothing but leave Jerry-shaped bends in the poker.
- Karma Houdini: Despite him starting the episode's conflict by stealing Tom's milk, nothing bad happens to Jerry throughout the episode (aside from several Oh, Crap! moments).
- Loud Gulp: Jerry loudly gulps when he learns the Super Serum's effects don't permanently effect him.
- Metronomic Man Mashing: Jerry flings Tom around in this manner while powered up by the poisoned milk.
- Miraculous Malfunction:
- Tom dumps a bunch of gunk in the milk—acid, ammonia, moth balls. But, instead of making the milk poisonous and killing Jerry, he creates an enlarging potion that gives him Super-Strength and invulnerability.
- At the end Jerry frantically tries recreating the potion as Tom is chasing him. Tom, triumphant, grabs the saucer of poisoned milk and drinks it. However, instead of turning into super-Tom he shrinks greatly. The cartoon ends with Jerry chasing him with a flyswatter.
- Murder Is the Best Solution: After Jerry's unrelenting attempts to drink Tom's milk, instead of drinking it in peace after snatching it back, Tom decides to use it as a means to poison him.
- Near-Villain Victory: Towards the end, Tom gets very close to defeating Jerry, but he lost after drinking his attempt at remaking his Super Serum which causes him to greatly shrink.
- Nigh-Invulnerability: When Jerry is in his super-mouse form, nothing Tom does to stop him has any effect at all. He attempts to whack Jerry with a book. The book breaks on Jerry's head and he tears it in half, then tears the other half into quarters. Tom whacks him with a poker four times. All this only results in is four mouse-shaped dents in the poker. Tom runs and shuts a door then braces it. Jerry simply walks it down. And when Tom tries hiding in the safe, Jerry simply drills it with his fist, yanks Tom out and bangs him against the safe like a ragdoll.
- Oh, Crap!: Jerry has at least four of them. Firstly, he's left surprised when he realizes the "magic milk" doesn't last forever for the first time, while Tom gives him a smug look. He then books it and drinks the milk again...but it barely lasts 5 seconds this time around, leading to this reaction again. It happens again when he runs to get the milk, only for Tom to quickly stomp on his bowl, preventing him from getting the milk again. The fourth and final one occurs when Tom traps Jerry after he's just a little too slow trying to make the poisonous milk again, and realizes Tom is about to drink it himself (which ends up backfiring spectacularly).
- Ominous Walk: Once Jerry becomes invincible, he power-walks towards Tom, the milk having made him an unstoppable menace.
- A Rotten Time to Revert: The potion's effects frequently wear off during the beatings (it seems to last longer the more you drink), reducing Jerry to his original size and forcing him to run away to top up his strength.
- Scare Chord: Merely a moment after the poisoned milk transforms Jerry into a hulking super mouse, we're treated to an ominous sting over the sound of Jerry's furious huffing. Then he starts chasing Tom, and then the soundtrack changes to a dark parody of the Superman radio show's theme.
- Screw This, I'm Outta Here: Tom tries escaping when he shrinks past Jerry's size. Unfortunately for him, the mouse stops him from doing so.
- Swallow the Key: After locking Jerry in another room, Tom swallows the door's key to try to keep him stuck there.
- Super Serum: The accidental magic milk potion, which turns Jerry into a super-mouse.
- Super-Strength: One of two powers the magic milk potion gives Jerry. He becomes able to rip a phone book in half, walk down a door and even punch through a safe, and throw Tom around like he's nothing.
- Tampering with Food and Drink: Tom tries killing Jerry by having him drink poisoned milk.
- A Taste Of His Own Medicine: At least a few times (one being from this episode), Tom has pinned down Jerry's tail. Here, after he shrinks past his size, Jerry does the same thing to him.
- Throw the Book at Them: After Jerry's first transformation into a super-mouse, Tom tries to whack him with a book. It fails.
- Villainous Breakdown: You'd start freaking out too if you shrunk past a mouse's size and your Arch-Enemy started chasing you with a flyswatter.
