Earth is invaded by super-intelligent flying brains, who sap the Earth's populace of their intelligence. Leela is taken to Nibbler's home planet Eternia, where the Nibblonians explain that only one human is immune to the brains' powers - Fry.

The crew's attempts to de-age Professor Farnsworth result in everyone returning to their more youthful stages. While Farnsworth seeks out a way to re-age the crew, the newly-teenaged Leela takes the opportunity to experience the parental childhood she never had.