The novel is set in World War I and opens with a framing story in which a manuscript relating the main story is recovered from a thermos off the coast of Greenland. It purports to be the narrative of Bowen J. Tyler, an American passenger with his Airedale terrier Nobs on a ship sunk in the English Channel by a German U-boat, U-33, in 1916. He is rescued by a British tugboat with another survivor, Lys La Rue. The tug is also sunk, but its crew manages to capture the submarine when it surfaces. Unfortunately, all other British craft continue to regard the sub as an enemy, and they are unable to bring it to port. Sabotage to the navigation equipment sends the U-33 astray into the South Atlantic. The imprisoned German crew retakes the sub and begins a raiding cruise, only to be overcome again by the British. A saboteur continues to guide the sub off course, and by the time he is found out it is in Antarctic waters.The U-33 is now low on fuel, with its provisions poisoned by the saboteur Benson. A large island ringed by cliffs is encountered, and identified as Caprona, a land mass first reported by the (fictitious) Italian explorer Caproni in 1721 whose location was subsequently lost. A freshwater current guides the sub to a stream issuing from a subterranean passage, which is entered on the hope of replenishing the water supply. The U-boat surfaces into a tropical river teeming with primitive creatures extinct elsewhere; attacked, it submerges again and travels upstream in search of a safe harbor. It enters a thermal inland sea, essentially a huge crater lake, whose heat sustains Capronas tropical climate. As the sub travels north along the islands waterways the climate moderates and wildlife undergoes an apparent evolutionary progression.