Homer has put on weight so Marge forces him to eat only one donut a day at work. Meanwhile, Mr. Burns wants to be younger, so he visits R & D to see how things are going with the rejuvenator ray, a machine that will make him younger. Now all they need to do is test out the machine on an employee. Through a security monitor, Mr. Burns observes Homer eating multiple donuts and chooses him to be his lab rat. The ray is fired at Homer but nothing happens and, the machine is assumed to be a failure.
At home, Homer tries on a new pair of jeans that don't fit him, so Marge must go to the store to buy the next size up. When she retunrs, she finds that Homer has become a monsterous behemoth. She manages to catch up to him but his colossal size make it impossible to hear her. While this is going on, the media is having a field day with this story and Mayor Quimby is on the war path after an embarassment he suffered at the hands of giant Homer.
As Homer continues his rampage, things are getting worse for the family, who are being hounded by the media. Not only that, Homer is heading toward the nuclear plant and the result could destroy Springfield, forcing Quimby to call the Pentagon for an air strike. Luckily, Mr. Burns wants Homer alive so he has Dr. Olberman, head of R & D cook up a drug that will render Homer unconscious and return him to normal size. However, only one batch has been made so they have only one chance. Lisa suggests using trucks off Duff Beer to lure Homer away from the plant and then inject him with the drug as he passes them. Their plan succeeds and Homer returns to normal.
Mr. Burns then runs a battery of tests on Homer to see if there were no long-term health effects from the whole ordeal. When the results come back, Homer and Marge learn that he has lost three pounds and that he is perfectly healthy. But when they leave, Smithers tells Mr. Burns that the results they heard weren't completely true, and that some of the side-effects of the ray turned him into a balding, obese, donut-obsessed buffoon.