Lisa fears she is losing her intelligence, and learns from Grampa that eventually all Simpsons lose their intelligence. So, throughout the episode, Lisa struggles to accept this. In the end however, we learn that this Simpsons gene only affects men, making the Simpson men of the family stupid, not females. So now Lisa is happy because she knows she will grow up to be an intelligent lady.
A good episode, but I don't like it as much as some other fans. I find some of it boring. My overall grade for this episode would be about a B- or so