Still not fully recovered from the excitement of finding that Enormous Egg, Nate Twitchell leaves his home in Freedom, New Hampshire to join his paleontologist friend Dr. Ziemer on an ex- pedition to the caves of France. Though Nate never warms up to the French custom of drinking wine with meals, he becomes handy enough with the language to make friends with a local boy named Nicol, and while Dr. Ziemer and the pompous Professor Newall are bogged down in a deadend investigation the boys discover a narrow passage leading to their own private prehistoric cave -- complete with wall paintings and a resident ""left over"" from cave man days. Nate and Nicol are kind of glad when their photographs of the cave's interior turn out blank and that they are never able to find the old man's placid ""little valley"" for Professor Newall. You may suspect, as Dr. Ziemer does, that that ""sharp-eyed,"" ""stringy-armed"" old codger in the cave was some kind of a hoax, but you'll never get Nate to believe that. He knows better, and considering he was right about the triceratops, he's a young man to put your faith in.