What is going on inside the spooky old Greely mansion? Peering into the deserted house, Kay Tracey sees a mysterious figure prying open the seventh step of the spiral staircase. Startled, the intruder flees through the dark halls and gets away. Kay, sixteen-year-old girl detective, has for the moment lost the prowler, but she finds herself head over heels in another baffling mystery. The chase is on!

What is hidden in the seventh step? The question sets Kay's sleuthing brain into high gear. Kay's cousin Bill, a young lawyer, becomes involved in the strange doings when he tries to sell the Greely estate. Even Kay's jealous schoolmate, Ethel Eaton, sees a chance to get even with Kay, with a scheme that ultimately backfires.

Not until Kay and her friends Betty and Wilma Worth uncover a strange clue do they get to the bottom of the exciting mystery.