A dam being built by Stephen Gray (Lafe McKee) comes under attack from the mysterious pilot, The Black Ace (Jack Mulhall), and his "Mystery Squadron" of pilots. The identity of the Black Ace is so secret that it is even concealed from his own men in the Mystery Squadron.
Fred Cromwell (Bob Steele) and his partner Bill Cook (Guinn "Big Boy" Williams) seek to identify the Black Ace and stop his attacks on Gray's construction of the power dam, as a number of characters behave suspiciously, including Lafe Johnson (Purnell Pratt), a rival contractor to Gray, Martin (Edward Peil, Sr.), the hotel owner, Collins (J. Carrol Naish), a construction employee for Gray and Dr. Flint (Robert Frazer).
