Paul Berna is the youngsters' Simenon: the accent aigu is on realism. The Thiriets live in a Parisian slum; stuffed in an apartment which is physically (and socially) difficult, they finally find nicer quarters and make a ten thousand franc down payment. But Mr. Thiriets has been had and the only lead to the confidence man who has taken them is a black cat-- actually two cats. The Thiriet youngsters, particularly Bobby and his friends, not only write up the case for their school paper but track the cat to the criminals... It is all handled with commendable integrity and intelligence.