After an unsqueakably fun summer at camp, Humphrey can't wait to get back to Room 26 and see all his familiar classmates. But something fur-raising happens on the first day of school: instead of his old friends, Mrs. Brisbane's room is filled with strangers. Humphrey wonders why his teacher won't tell them the truth: they're in the wrong room!
While Humphrey gets to know the new students, he wonders what happened to his old friends. Where could they be? What could have happened to them?
It's a big mystery for a small hamster to solve, but little by little, his nightly adventures help him piece together the answer and he sees a dark side of his school he's never seen before.
There's a whole lot of shaking going on in Room 26 as well, with the mysterious new Rockin' Aki ... Mrs. Wright and her whistle ... and the worry that maybe, just maybe, Humphrey is going to be replaced!
Meanwhile, Humphrey has his paws full as he learns that these strange students need a helpful classroom hamster just as much as his old friends ... maybe even more!