SpongeBob visits his Grandmother's house, where he acts and is treated like a small child, engaging in such babyish activities as eating cookies and being read stories. As soon as the clock strikes 3:00 p.m., SpongeBob has to go back to work and is given a ride there by his Grandma. When they get there, she kisses him goodbye, leaving a kissy mark that is visible on his forehead.
SpongeBob is then teased by everyone at the Krusty Krab, and runs home crying. At home, SpongeBob consoles Patrick about this, and Patrick tells him that he should show his Grandma that he is not a baby anymore by acting like an adult. SpongeBob and Patrick go to Grandma's house wearing fake sideburns, and when she answers the door and tries to kiss him, he refuses to be kissed, explaining to her that he is an adult now.
Grandma offers them cookies, which SpongeBob refuses, but Patrick gladly accepts. Patrick begins acting like a baby and when SpongeBob confronts him, he says that being a "grown-up" is boring and that he likes being a baby. SpongeBob attempts to eat a cookie, but Grandma stops him, instead giving him steamed coral from his grandma. Then she tells Patrick a baby story and gives SpongeBob a giant technical manual; later she gives Patrick a sweater "with love in every stitch", and gives SpongeBob unwrapped office supplies. Soon, the clock strikes 3:00 p.m., time for SpongeBob to go to work.
SpongeBob starts walking toward the door and starts to leave as his Grandma lays Patrick down for a nap. Before he leaves, he talks to Grandma, who shushes him because Patrick is asleep. However, SpongeBob soon returns, crying and saying that he doesn't want to grow up, and wants to become a baby again and do all the stuff he did when he was treated like a baby like eating cookies, drink out of a bottle with milk, wear a sweater "with love in every stitch", wear diapers, ride in his wagon, cuddle with a teddy bear named Mr. Stuffykins, rocking on his seahorse, and get a kiss on his boo-boo. He then cries so intensely that he floods the house.
But Grandma calms him down and pulls a drain to remove the water. She tells SpongeBob that he doesn't have to be a baby to get her love, and that he will always be her baby boy no matter how grown up he gets. She gives him a cookie and hugs him. SpongeBob asks her to "not mention this to the guys down at the Krusty Krab", to which she replies "No problem." Then Grandma gives SpongeBob a hug. However, Squidward and the people from the Krusty Krab are standing outside the window and watching this, and start laughing as the episode ends.
SpongeBob and Patrick in a deleted scene from storyboard
In the final scene, the kitchen changes locations.
When SpongeBob says "I don't?", his reflection looks like his mouth doesn't move and was open as if he were still crying.
When Patrick uses his mouth as a vacuum cleaner to eat his cookies, one can clearly see that he's eaten all of them, but one rolls out.
When Squidward and all the customers are seen laughing at SpongeBob hugging his grandma at the end, Harold was green instead of blue.
One fish was talking to SpongeBob and at one part, his arm wasn't on his body.
A glass of milk and a plate with two cookies are shown on the table when Patrick says that he doesn't get jazz, though in the next shot when SpongeBob starts blowing bubbles out of his pipe, they were both gone.
In first shot Clayton was seen blue, in second shot he was seen purple, and in next shot, he was blue again.