Arthur's friend Buster is searching for a crime to solve. So when the quarters Arthur has collected for Mrs. MacGrady's charity drive mysteriously disappear, Buster is committed to cracking the case. Will Buster be able to prove Arthur's innocence so that he can attend the class picnic?
Francine needs a new bike, but all her parents can give her is her dad's old one...and it is ancient. When the bike disappears, Francine thinks her problem is solved, but she soon finds herself anything but relieved.
After Arthur calls Francine a marshmallow, she refuses to speak to him. Then Mr. Ratburn pairs them up for a homework project. They reluctantly go to the library for research and, when the library closes, find themselves locked in. Will Arthur and Francine set aside their differences and work together to find a way out?