3x13 - Maniac Mansion

Fred dreams that he's starring in a 50's science-fiction movie in which he is a brilliant scientist, Casey is bed-ridden and blind, Ike is a normal 50's kid and Tina is a brainy bombshell. There is a giant beaver terrorizing the Eastern United States and Fred, in consultation with President Eisenhower, develops a plan to defeat it, using the A-Bomb and rock star Brad Stones. The plan works, and the resulting A-Bomb blast cures Casey's blindness. Then Fred wakes up and realizes that it's back to his normal everyday existence.