Arroyo Seco (""dry canyon"" in Spanish) is the scene of a somewhat routine mystery. Willie and friend Joe Pike are helping Don John- a Spanish Indian- dig for family treasure in a ravine near Los Angeles. The ambitious hunt is sidetracked when it hits smack up against the rambling house of old Mrs. Goodfellow. The subsequent appearance of one of the Reachis (a notorious group of bandits) in the vicinity sends Don John straight to the police where he meets with skepticism. On their own, the little group discovers that Mrs. Goodfellow is being kept prisoner in her cellar while the escaped criminal has full run of the house. The ingenuity of all concerned combine to capture the villain, free the old lady and find the treasure. There are touches of humor here -- but the overworked theme and pat ingredients pre-empt suspense and make this predictable fare.