Proof that a small mystery of small pretensions can work well in its small way. The Mariner Troop of thinly sketched sixteen and seventeen-year-olds is quickly put aside when two of its members--energetic, natural Jo and clumsy, failure-prone Molly--spin off to a smaller Bahama island for a week. The inhabitants are a stock mystery crew-Mrs. Randolph who is really Miss Nelson and has locked the real Mrs. Randolph in the secret room, a two-faced reasure hunter, his violent assistant--plus one unpredictable--an enigmatic young diver friendly with Jo. With his help the girls figure out the captain's deception, decipher the clue to the secret room and handily bring all to a happy close. Fast-moving and comfortable for a few hours.