At the end of The Thirty-nine Steps, Hannay tries to find the man who posed as the First Sea Lord by remembering Scudder's book, in which Scudder wrote down that the enemy would try to escape when the high tide takes place at 10:17 at night and where there are 39 steps. The man who pretended to be First Sea Lord has secret knowledge of England's military defenses that could be delivered to England's enemy. To stop him, Hannay tries to find a small port in which the high tide will occur at 10:17 at night. He determines the enemy is leaving from a small port because the high tide will be important, and the enemy can only leave this type of small port when it is high tide.
Hannay finds a place called Bradgate in Kent that meets this description. When he goes there, he finds a house called Trafalgar Lodge that has 39 steps. He notes a suspicious yacht in the harbor, and when he goes to the house that night to arrest the three men inside, he feels at first that he has made a mistake. The men seem perfectly English, not like the German enemy. However, when one man begins to tap his hand, Hannay realizes he has seen this man before and that the man is a member of the enemy he met on the moorland farm. Hannay captures two of the men with the police, and the other man, who tries to flee to the Ariadne, the yacht in the harbor, is caught by the police. Three weeks afterward, England goes to war, and Hannay becomes a captain. He feels, however, that his greatest service to his country has been in stopping the Black Stone.