Dr. Jekyll and Mr. Mouse is a color Tom and Jerry cartoon that was released to theaters on June 14, 1947.
The short opens with Tom the cat sneaking one of the bottles of milk that the milk man delivers to the house that Tom lives in into the living room so he can enjoy it. However, a hungry Jerry the mouse decides to sneak a sip or two from the bowl that Tom poured it in. However, Tom tries to get rid of Jerry when he notices him, but to no avail. So, Tom decides to taint the milk by combining different household chemicals with it, including poison, bleach, and mothballs. The poisoned concoction proves to be very effective when a fly lands on the edge of the bowl and takes a sip, and then drops dead. Tom takes the poison drink and puts it right outside of Jerry's mouse hole.
Jerry notices and smells the milk, thinking it might be a trap, but ends up drinking most of it after just one sample. The potion seems to take effect and Jerry falls dead. Tom smiles with wicked glee, but soon notices something else happening. Turns out that the tainted milk did not kill the mouse, but had a Jekyll and Hyde effect on him by turning him into a muscular hulk. Tom panics as the monster mouse walks toward him, and nothing he hits him with phases him in the least. However, the potion wears off and Jerry turns back to normal.
Tom prepares to carve Jerry up, but the potion activates again and Jerry becomes a monster. This time, he punches his way through a safe that Tom is hiding in to get to him. But just when he is about to do his worst, the potion wears off again, for good this time. So Jerry escapes before Tom can do anything, and heads to the basement to recreate the potion. However, Tom gets to the basement and drinks the whole thing. He starts to grow, but then he begins to shrink down to Jerry's size.
Both of them soon realize that every time Tom gets hit by something, he shrinks smaller and smaller. Jerry then proceeds to chase the now tiny Tom around the house with a flyswatter.