Being a classroom hamster means having to be ready for anything, but suddenly there are a lot of big surprises in Humphrey's world. Some are exciting, like seeing the playground during a fire drill and getting to roll around in a hamster ball. But some aren't so good, like a cat creeping around, a different janitor who might be from another planet, and the most shocking thing of all -- the possibility of Mrs. Brisbane retiring. Even with all that's going on, Humphrey makes time to help a few classmates with their problems. But could Mrs. Brisbane leaving school be one problem that's just too big for this little guy to solve?