Homer goes to a costume party with Marge as Radioactive Man, gets bonked on the head and thinks he is Radioactive Man.
Homer doesn't like his costume for the Mayor's Costume Ball as he ends up being Radioactive Man while the Lovejoys steal the last Star Wars costume. Homer wanted to be a yummy tart but the Mayor makes Luanne Van Houten the winner by saying the winner is "this yummy little tart". Homer grumbles that should have been him, Kirk gets mad at the mayor for making a pass at his ex-wife (at the time) and gets carried off. Homer ends up getting a big bump to the head and is unconscious.
Homer wakes up at the hospital and sees his costume and becomes Radioactive Man. Bart thinks it's great to see Homer in the getup, hoping to get on Ricki Lake while Lisa shoots him down. Homer gets stopped by the police, who Marge tells about Homer's costume but Chief Wiggum mistakes her wording for the fact he is radioactive. The police see the costume and put him in the same mental hospital as in "Stark raving Dad". Marge is upset that the police would do such a thing and can't get Homer out if she tries cause all the Simpson's money is tied up in getting detectives to find Grampa in South America.
Homer bumps into Leon (the guy who thinks he's Michael Jackson) and as Radioactive Man and Fallout Boy they save Springfield somehow. Meanwhile, Marge is forced to take over Homer's job and falls asleep easily but after a big issue at the plant, she is saved by Homer and Leon. Unfortunately, Leon gets a bump to HIS noggin and thinks he is Bart, Marge lets out a "D'oh" moan.