The Simpsons take a trip to the Family Fun Center. On the go-kart track, Marge sees Nelson run Milhouse off the road. Later, in the arcade, Bart sees Nelson cash in his prize tickets for a BB gun. He asks Nelson if he can come over and try it sometime. Marge overhears this and, having witnessed Nelson's ealier behavior, forbids him to do so.

Neverthless, Bart goes to Nelson's When he tries out the gun, he accidentally kills a robin. Marge finds out, and instead of handing down a punishment, she simply tells Bart that she is giving up on him. Bart discovers that the robin had eggs in her nest and decides that he can't let them die, too.

When Marge finds out that Bart is trying to save the robin's eggs, she forgives him. When they hatch, that family discovers that the nest actually contained lizard eggs. Marge, Bart, and Lisa take the lizards to the Springfield Birdwatching Society to find an explanation. There, Seymour Skinner informs them that what they have are Bolivian Tree Lizards, and deadly spaecies that consumes bird eggs without the mother's knowledgs and lays their own in the nest. He informs Marge and Bart that the lizards have to be killed. Marge helps Bart escape with the lizards, but the birdwatchers catch up with him. After a struggle, the lizards are accidentally set free. They go on to decimate the town's pigeon population, to the delight of the townspeople.