Jerry is asleep in his bed, dreaming of Tom catching him and knocking him on the head. The mouse wakes up and hears the knock again, to his fright, but sees it's actually a knock at his door and goes out to open it. There is basket on his front door; Jerry does not notice when he peeks out the first time, and then widens his eyes in puzzlement. He pokes the basket, and it comes alive, runs in circles through Jerry's home, and then runs into a wall. Jerry pulls the blanket off to reveal it is Nibbles. Wondering what to do with him, he picks up the note attached, which says, “Please take care of Little Nibbles---He’s always hungry! P.S. Feed him lots of milk". Nibbles then confirms he's hungry by licking his lips and rubbing his belly.
Seeing that he has no food anywhere in his hole, Jerry tells Nibbles to be quiet and he tiptoes out of his hole, spotting Tom asleep by his bowl of milk. Nibbles sees the milk and runs toward it; Jerry repeatedly grabs the smaller mouse and pulls him back into the hole until he is forced to hang Nibbles on a hook by his diaper. Jerry then tells Nibbles to stay put, and goes out to the kitchen to get the milk for his younger protege.
Meanwhile, the disobedient Nibbles has caught up and passed Jerry. The bigger mouse doesn't notice until Nibbles climbs on Tom’s nose and jumps onto the rim of Tom’s bowl. Nibbles loses his balance and has to hang onto one of Tom’s whiskers, which causes Tom to wake up. Before the cat can open his eyes and see them, Jerry grabs Nibbles and the two land in the milk, splashing it in Tom's face. 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.
In horror, Jerry jumps out of the milk and bluntly opens the cat's mouth to retrieve Nibbles, which begins the chase. Jerry is holding Nibbles as he runs, but Tom starts to catch up, so Nibbles switches positions until they reach the mousehole. Nibbles is safe, but Jerry bumps his head on the wall just above the mousehole. Fortunately, Nibbles quickly pulls him into the hole before Tom crashes headfirst into the wall.
With the direct attempt having failed, Jerry builds a pipe bridge to the milk by stringing straws together all the way into the bowl. However, Tom is alert and opens one eye to see the end of the straws land in the bowl, and to hear and see milk being sucked out of his bowl. Knowing this is a hungry Nibbles, Tom turns the tables by grabbing the other end of the straw and drinking through it, sucking Nibbles through the straws in an attempt to eat him. Seeing the rapidly-worsening situation, Jerry squishes the very end of the straw, saving Nibbles just in time, and blows the small mouse into the mousehole.
Extremely angry at this turn of events, the cat chases Jerry into his hole until Tom moves the hole over so that Jerry crashes into the wall. Tom grabs Jerry, but Nibbles comes out to make the save by hitting Tom’s tail with a hammer. The chase starts up again until Jerry signals for Tom to stop, while Nibbles sips and spits some milk into Tom's face.
Jerry and Nibbles hide in a closet, while Tom hammers on the door. Seeing a chance to use deception, Jerry uses a container of shoe polish he discovers to disguise himself and Nibbles as a pair of black people. Emerging from the closet, Jerry, dressed in headscarf and in a blackface and he 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, eventually leading to Jerry grabbing a frying pan and hitting Tom in the face with it, knocking him out.
Jerry sadistically 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 allows himself to be chased through the room repeatedly. However, instead of Nibbles whacking the cat when the pair are in range, he 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 along with Jerry. Tom grabs a bottle to trap the mice in as they emerge, but picks it up too soon and collects only Jerry in the bottle. Nibbles taunts the cat and darts back under the rug, while Tom puts a stack of books on top of the bottle and goes under the rug to get Nibbles. However, as soon as he emerges, he meets a pie placed at the exit by Nibbles.
The cat chases him until he stops at Tom’s bowl again, signals for the cat to stop, and starts to drink from it. Tom, after allowing the mouse to drink briefly, grabs Nibbles. But when he opens his hands, Nibbles has crawled under Tom’s fur. Therefore, Tom grabs his tail and a pistol and shoots at Nibbles when he emerges from it, but just shoots his tail instead, which, of course, causes him to scream in agony. As Nibbles runs away, the cat chases him with a flyswatter and successfully corners him. He gives him a good swat on the behind, Jerry who runs directly through the glass, popping it, to inspect his nephew.
Tom's swat left a big red mark... and Jerry is furious about it. He faces Tom with no mercy. Seeing he has pushed Jerry too far, Tom pathetically hides the flyswatter behind his back, and Jerry, not fooled in the slightest, lets out a massive bull roar. This makes Tom literally turn yellow with fear as Jerry attacks him, slamming the cat against the floor three times, throwing him behind a garbage can and popping open the lid into the cat's face repeatedly.
In the final scene, Jerry is shown standing over Tom's shoulder with a wooden meat tenderizer keep the cat in line under threat of being whacked while a bandaged Tom spoon-feeds some milk timidly to Nibbles as punishment for having terrorized the young mouse.