When Arthur's third-grade class goes hunting for fossils at Rainbow Rock State Park, his friend Buster actually finds one, and instead of turning it in Buster decides to take it home--but he soon regrets his decision.

Buster goes away for a month, and Arthur becomes sad and confused when he does not receive even one postcard from him. When Arthur's best friend Buster leaves town to visit his father for a month, Buster promises to send postcards to Arthur. Every day Arthur looks for a postcard in the mail, but finds nothing from Buster. When Arthur learns that all his friends at school have heard from Buster, Arthur is crushed-until the postman sets things right...

Buster sends postcards to his friends back home when he and his father go to San Antonio, Texas for the International Accordion Festival.

When his father takes him to visit Vermont, Buster sends postcards to his friends back home telling them what he is learning about maple syrup and the "mud season.".

Buster and his father visit South Dakota and send postcards home describing their visit to the Crazy Horse monument, their experience with buffalo, and more.