Sons of the Sea (1939)

The head of Dartmouth Training College has been murdered, and his successor, Captain Hyde, suspects that he himself may have been the intended target. He enlists the help of his own son Philip, a reluctant cadet at the college, to help him confirm his suspicions about planned enemy action... but meanwhile, there is a Secret Service agent staying at the college observatory, the foreign-born professor of astronomy is behaving strangely, and Naval Intelligence disclaim any knowledge of what is going on...