CONTENTS[CHAPTER I. A MYSTERY][CHAPTER II. ON ACTIVE SERVICE AGAIN][CHAPTER III. A LESSON][CHAPTER IV. STRANGE WARFARE][CHAPTER V. THE END OF BLOSBERG][CHAPTER VI. WITH THE MOTORBOAT FLEET][CHAPTER VII. AN ENEMY][CHAPTER VIII. FRANK IN TROUBLE][CHAPTER IX. JACK TO THE RESCUE][CHAPTER X. LOST—THE STORM][CHAPTER XI. ON THE HUNT][CHAPTER XII. THE FIRST VICTIM][CHAPTER XIII. A DESPERATE VENTURE][CHAPTER XIV. WITH THE ENEMY][CHAPTER XV. PLOTTING][CHAPTER XVI. AN ENEMY SUNK][CHAPTER XVII. AN ARGUMENT SETTLED—TEMPORARILY][CHAPTER XVIII. MISTAKEN FOR AN ENEMY][CHAPTER XIX. THE TRAP IS SET][CHAPTER XX. THE TRAP IS SPRUNG][CHAPTER XXI. AT SEA AGAIN][CHAPTER XXII. TOWARD OSTEND AGAIN][CHAPTER XXIII. AN UNEXPECTED ENCOUNTER][CHAPTER XXIV. ANOTHER UNPLEASANT SURPRISE][CHAPTER XXV. DOOMED TO DIE][CHAPTER XXVI. MORE TROUBLE][CHAPTER XXVII. THE PIRATES REAPPEAR][CHAPTER XXVIII. THE BATTLE AND THE ATTACK ON THE U-6][CHAPTER XXIX. THE END OF A TRAITOR]