2x14 - Maniac Mansion

Casey's parents are having a party for their 50th wedding anniversary. Everyone's going except for Fred, who sprained his toe and is stuck in bed. Right before they leave, a woman comes to the door and claims to be Casey's biological mother. After talking to her, Casey decides to go to her other parents and leave Fred with her mother at home. Fred starts noticing Dolly is acting strange, and later figures out that she's crazy. He attempts to knock her out, but she ends up noticing him and they start to fight. At that same moment, Casey and the rest of the family come home and break it up. It turns out that the woman had amnesia and thought she was Casey's mom.