In the American Southwest, Major Dennis, Danny, and Rex witness an atom bomb explosion. They meet a girl paleontologist. Another test is scheduled for tomorrow -- Suddenly a Tyrannosaurus Rex attacks! Major Dennis orders they run while he attacks with his car and pistol. Danny, Rex, and the girl run into a lost valley uncovered by the A-bomb. They survive a giant clam and Pterodactyl, see a Brontosaurus battle a Stegosaurus - and escape the next day. But the bomb is due to go off! They find Major Dennis alive and hide in a cave as the bomb explodes. The T-Rex and valley are gone.; There's a reward out for counterfeiters. Danny shows his money to Mr. Neilson at the bank - and Rex growls at the friendly teller! The police lock Rex up for a medical exam. Danny follows Mr. Neilson to the deserted Stone Elephant Hotel - and is locked in! Rex breaks free, tracks Danny to the hotel, and bowls over a false Mr. Neilson. With Danny and the real Neilson free, they find a hidden passage leading to the counterfeiters! The fake Neilson used the bank to pass fake money. Rex takes them down, and Danny gets a reward so his Mom can take a vacation to Europe!; While the Sheriff chases a racketeer named Danny Sharp, Bobo goes with a female friend to the track to see Jubilee, her race horse. Someone drops a stick of dynamite that Bobo hurls away. When gangsters torch the stall, Bobo rides Jubilee to safety and hides him. Finally he climbs a palm tree and beans the bad guys with coconuts - and gets a jockey suit as a souvenir!