Pepi is a poor boy who became Homer's little brother.
When Bart joins the Bigger Brother program and is assigned to Tom, Homer, in revenge, signs up as well and gets Pepi. As Homer is his big brother, he learns nonsensical things, such as that whales are not mammals. As Homer and Tom get into a fight, Homer is sent to hospital, leaving Pepi without a bigger brother. Bart sees that Pepi is more deserving of Tom than he is, so he pairs them up, and they walk into the sunset.