The Simpsons take a trip to a family fun center, where Nelson wins a BB gun. He invites Bart to come over and try it out sometime, but Marge dislikes the idea. Ignoring his mother, Bart goes to Nelson's where he accidentally shoots a bird. Marge finds out, and tells Bart she is giving up on him.
Bart discovers that the bird left behind two eggs, and decides to look after them. When the eggs hatch, lizards emerge, and Bart takes them to a local bird watching society to find out why. Principal Skinner explains that they are Bolivian Tree Lizards who are making certain species of bird extinct and must be killed.
Upset, Bart runs off with the lizards and the birdwatchers follow. A struggle on the roof leads to them being set free. However, they turns out to be a godsend when they decimate the pigeon population.