During a fierce mountain thunderstorm at the Bleeker family's vacation farmhouse, there appears a stray cat. Not just an everyday, scraggly old stray, but a magnificently striped, well-fed, green-eyed, mysterious--very mysterious to the Bleeker kids--b-i-g c-a-t. Where did it come from? To whom does it belong? Can Ray, Trina, Angus, and little Kate, keep it? Always Absolutely not, their Father informs them. They may keep it for just two days while they try to locate the owner, and failing that, it's the A.S.P.C.A. for Amnesia. (For that is what they named the cat, because he'd forgotten where he came from.)

So the kids, working under under the strict two-day deadline, set out to find Amnesia's owner. Gathering clues from his behavior and habits, and faithfully putting everything down on Angus's tape recorder, they scour the neighborhood, bike to distant towns, interview dozens of people, all without success. Until the strange behavior of the "trailer lady" convinces Trina that they are on to something at last...and it may be murder!