Marge puts Homer on a diet for being too overweight, and so he is forced to go without donuts until he loses ten pounds. At the Nuclear Power Plant, Mr. Burns is in his secret underground lab preparing to test Project Y, a youth ray that will make Burns young again. Since the workers' union has given Burns the right to perform biological experiments on them in exchange for an extra 10-minute coffee break, Homer is picked at random and hit with the ray to seemingly no effect.
The next day, Homer tries to get dressed only to find that none of his clothes are fitting. Before Marge or anybody else knows it, Homer grows into a giant and marches through Springfield in his underwear. As everybody in the city responds in various forms of panic and awe, Homer is oblivious to his new size and everything around him as he heads to work, unaware that he'll destroy all of Springfield when he steps on the power plant.
The rest of the Simpsons meet with Mayor Quimby, Chief Wiggum, and Mr. Burns in City Hall's war room. Burns' chief scientist Dr. Olberman has developed a serum to shrink Homer, but he only has enough for a single dose. On Lisa's suggestion, beer trucks are driven past Homer to distract him long enough for Olberman to hit Homer with the serum, restoring him to normal size.
After a series of follow-up tests, Homer is returned to his family, and he is also taken off his diet when the results show that he has lost ten pounds . When Mr. Burns demands that he be turned into a giant, however, Smithers tells him that the ray has a horrible side-effect: anybody who is hit by it turns into a balding, obese, donut-obsessed buffoon. Cursing this unfortunate news, Burns orders that research continues. However, he fails to notice Homer's giant head peering into his office.