His revolver he found now to be of no use, so he dropped it and struck out blindly with his bare fists. Once, twice, his fists found their mark, and each time a blow went home the lad was rewarded by hearing cries of pain from his opponent. As the two struggled, there flashed before the lad a vision of a man running from where the D-16 lay in drydock some days before. "I guess we have got him at last," the lad muttered between his teeth, and putting all his force behind one more blow, he struck out savagely. The arms about his neck relaxed their pressure and the man sank to the floor while Frank felt the form grow limp beneath him. The lad stood up and walked across the little room to snap on the searchlight. As he did so the man on the floor came quickly to his feet, and before Frank could stop him, had darted from the room and disappeared. Through the door he left open streamed a faint light. Frank sprang after the retreating figure with a cry of anger. Dashing out of the door he bumped squarely into the figure of another man advancing toward him. Without pausing to see who the newcomer might be, Frank grappled with him. "Here, here, what's the meaning of this?" asked a well-known voice, and Frank released his hold and stepped back. The newcomer was Lord Hastings. "What's the matter with you?" asked the commander of the vessel. "Have you gone crazy? I stopped the ship in response to your command and when I asked you, through the tube, what was wrong I didn't get an answer. And now you jump on me. What's the matter?" "Spy aboard, sir," replied Frank briefly. "What!" exclaimed Lord Hastings, starting back. "Spy aboard, sir," repeated Frank. "Impossible!" exclaimed Lord Hastings. "Have you lost your senses? How could a spy have got aboard?" "As to that I don't know," replied the lad, "but nevertheless there is a spy aboard. I'll stake my life on that." Then he proceeded to relate what had occurred. Lord Hastings grew very grave. He took Frank by the arm. "Come with me," he said quietly. He led the way to his own cabin, where he passed the word for Jack. The latter arrived almost immediately, and the situation was explained to him. "I believe," said Frank, "that the man is the same I saw lurking about the ship yard before the D-16 was put into commission. "So you saw him, too?" exclaimed Jack. "Yes, I saw him, but I didn't know you did," replied Frank. "I didn't say anything because I thought I must be wrong," said Jack. "It didn't seem possible a German spy could have gained admittance there." "Just what I thought," said Frank. "Do you suppose the man who attacked you just now is one of the crew?" asked Lord Hastings. "Who else could he be, sir?" "But I could have sworn by the members of my crew. They have all been in the service for years and are British to the backbone." "There must be one who isn't," said Frank, "for when I struck him he let out a stream of German oaths." "Would you recognize him?"