The story of the First World War is almost always told through the eyes of men in the trenches. But there was another war, just as cruel - the war at sea. And among the men who fought it few were more heroic than the officers and crews of the navy's secret fleet of Mystery Ships. A Combat of Devils is a gripping tale of reckless courage, based on the real-life exploits of seamen who fought back against the marauding U-boats. Early summer 1915. Britain's sea routes are a killing zone for the Kaiser's submarines. The Royal Navy's Special Service deploys a new weapon; cargo vessels carrying hidden guns, crewed by volunteers who offer themselves as bait to lure the enemy into a trap. After an heroic action, Julian La Salle is given command of Freya, a trading schooner converted to a Mystery Ship and sent to hunt down a submarine prowling the sea lanes off Ushant. With him is a young American, Merrick Granger a survivor from the Lusitania who volunteers for the war to escape a painful past. La Salle and Granger become particular friends, bonding closer after their ship is torpedoed off the French coast and they, with two other survivors, take refuge at the home of di Fario, a boatyard owner and veteran of the Franco-Prussian war. Di Fario lives with his daughter, Laurentine, to whom Granger and La Salle - in different ways - are attracted. When the wreck of Freya is found on a reef, La Salle hits on an audacious idea to strike at the U-boat. It is a desperate and possibly suicidal plan but di Fario, haunted by a tragic secret, is determined to be part of it and so too, is his daughter. Driven by love, Laurentine is plunged into the horror of war in a way none of them could have foreseen.