Is Mrs. Krantz' visiting Hungarian grandnephew a werefrog? The woman first hires McGurk's organization as babysitters, but soon admits that she really wants them to investigate the oversized frog noises that have been terrifying her ever since seven-year-old Bela came to stay. Mrs. Krantz' frog phobia is longstanding, but even the kid detectives are scared silly when the supposedly sleeping Bela vanishes and a large frog with the same soulful eyes stares back at them from Bela's pillow. It takes an official-looking search warrant, a microphone, tape recorder, and lie detector (all rigged up by the agency's ten-year-old wizard Brains Bellingham), some library research on the Cuban tree frog (whose voice, it is written, is ""a very loud, snarly rasp""), and some psyching out of cool little Bela--but at last the McGurk gang exposes the presence of Bela's smuggled-in ""best friend"" Fidel, then turns in to build a cage for the frog so that Bela will he allowed to keep it. This time around the McGurk formula works like a charm, with some real surprises, intrinsically funny scenes, and a realistically small-sized mystery that nevertheless teases for solution.