When Arthur's third-grade class goes hunting for fossils at Rainbow Rock State Park, Buster actually finds one. The problem is, he wants to keep it. Can Arthur persuade Buster to return the important discovery, or will it stay buried forever in the back of Buster's closet?
It looks like trouble when the principal asks Arthur to take home a large envelope marked CONFIDENTIAL. His friends try to guess what's inside. It just might be information on ... summer school! Will Arthur face the consequences or pretend to lose the envelope?
When Buster defeats the Brain in a math contest, he is chosen for the school mathathon. Crushed, the Brain says he's decided to become a comedian and declares Buster the new "brain."