The Milky Waif

Jerry is asleep in his bed having a nightmare when he wakes to the sound of a knock at his door and goes out to open it, finding a basket on his door step. He pokes the basket, and it comes alive and runs around in circles. Jerry pulls the blanket off to reveal Nibbles. An attached note says, "Please take care of Little NibblesâHe's always hungry! P.S. Feed him lots of milk." Nibbles then begins licking his lips and rubbing his belly.
Seeing that he has no food, Jerry tiptoes out of his hole, spotting Tom asleep by his bowl of milk. Nibbles sees the milk and runs toward it, but Jerry pulls him back and goes out to get the milk for him. Nibbles runs out, accidentally waking Tom. Jerry grabs Nibbles and the two fall into the bowl of milk, splashing it in Tom's face and waking him. Tom, startled, looks around and sees no one, so he starts to drink his milk. Jerry and Nibbles are under the milk holding their breath until Tom unsuspectingly scoops up Nibbles with his tongue. Jerry jumps out of the milk and opens the cat's mouth to retrieve Nibbles, which begins the chase. The mice retreat into the mousehole before Tom crashes headfirst into the wall.
Jerry builds a pipeline of straws to the milk but Tom turns the tables by grabbing the end of the straw and sucking Nibbles through it. Jerry squishes the end of the straw, saving Nibbles just in time, and blows the small mouse into the mousehole. Tom chases Jerry back to his hole but the cat moves the hole over so that Jerry crashes into the wall. Tom grabs Jerry, but Nibbles comes out and hits Tom's tail with a hammer. The chase starts up again until Nibbles sips and spits some milk into Tom's face.
Jerry and Nibbles hide in a closet, while Tom pounds on the door. Jerry uses a container of shoe polish to disguise himself and Nibbles as a pair of black people. Jerry (voiced by Lillian Randolph) emerges from the closet dressed in a headscarf and in blackface and greets Tom. Out comes Nibbles, also in blackface and wearing a headscarf. However, the disguise is revealed when Nibbles' diaper falls down, exposing his tail and gray fur. The chase continues until Jerry hits Tom in the face with a frying pan, knocking him out.
Jerry pulls out Tom's tongue, and with all his strength knocks the cat in the head, causing him to chomp his tongue. Tom yells and the chase begins again. Jerry hands the frying pan to Nibbles and continues leading the chase. Instead of whacking the cat, Nibbles swings too soon and smashes his comrade. This repeats a second time, but on the third time around, Jerry stops and points at the cat. Nibbles obliges and hits Tom square in the face, then crawls under the rug to escape with Jerry. Tom grabs a bottle to trap the mice but catches only Jerry. Nibbles taunts the cat and darts back under the rug, and Tom continues the chase.
Nibbles stops at the bowl again, signals for the cat to stop, and starts to drink from it. Tom allows the mouse to drink briefly, then grabs Nibbles, but Nibbles crawls under Tom's fur. Tom grabs his tail and a revolver and shoots at Nibbles when he emerges from it, but just shoots his tail instead, screaming in agony. As Nibbles runs away, the cat chases him with a flyswatter and corners him. He gives the mouse a great swat on the back, and an enraged Jerry breaks through the glass to inspect his nephew. Jerry is furious about a big red mark left by Tom's swat. The guilty Tom hides the flyswatter behind his back as if to say, "It's not what it looks like," but Jerry, not fooled, lets out a colossal roar, Tom turns pale with fright. Tom tries to run away but Jerry grabs his tail, slams him against the floor, then throws him behind a garbage can and smashes the lid into Tom's face repeatedly.
In the final scene, a still-enraged Jerry stands menacingly with a wooden meat tenderizer over a battered and bruised Tom to keep him in-line under the threat of getting whacked, wanting him to feed Nibbles milk as punishment for terrorizing the young mouse. Tom timidly spoon-feeds milk to Nibbles, not to let one drop spill or else he would be hammered by Jerry.

Question: What wakes up Jerry?
Answer: the sound of a knock at his door