A manuscript is recovered from a thermos off the coast of Greenland purporting to be the journal of Bowen J. Tyler, an American passenger on a ship sunk in the English Channel by the U-33, a German U-boat, in World War I 1916. He is rescued by a British tugboat with a female survivor, Lys La Rue. The tug is also sunk, but its crew manages to capture the German submarine after it surfaces. Unfortunately, the remaining 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 discovered, the U-boat is in Antarctic waters. The U-33 is low on fuel, with its provisions poisoned by the saboteur Bensen. A large island ringed by cliffs is discovered and identified as Caprona. A freshwater current guides the U-boat to a stream issuing from a subterranean passage, which they enter with 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 Caprona’s tropical climate. As the sub travels north along the island’s waterways the climate moderates and wildlife undergoes an apparent evolutionary progression. On the shore of the lake the crew builds a palisaded base, dubbed Fort Dinosaur for the area’s prehistoric reptiles and fauna. The British and Germans agree to work together under Tyler, with Bradley, the mate from the tug, as second in command and Von Schoenvorts, the original sub commander, in control of the Germans. The castaways are attacked by horde of primative beast men and take prisoner Ahm, a Neanderthal. They learn that the native name for the island is Caspak. Oil is discovered, which they hope to refine into fuel for the U-33. As they set up operations, Bradley undertakes various explorations. During his absence Lys disappears and the Germans mutiny again, absconding with the submarine.

Tyler leaves the other survivors to find and ultimately rescue his British love interest. Reunited and stranded in a LAND THAT TIME FORGOT…

"...I have experienced a cosmic cycle, with all its changes and evolution’s for that which I have seen with my own eyes in this brief interval of time-things that no other mortal eye had seen before, glimpses of a world past, a world dead, a world so long dead that even in the lowest Cambrian stratum no trace of it remains. Fused with the melting inner crust, it has passed forever beyond the ken of man other than in that lost pocket of the Earth whither fate has borne me and where my doom is sealed."

Thus begins one of the most thrilling science-fiction series that the creator of Barsoom and Tarzan ever conceived-the epic of the lost continent of Caprona and of the American who first set foot there.

I could see the surface of the water literally black with creatures of some sort... the general impression was of a vast army of amphibious monsters. The land was almost equally alive with crawling, leaping, running, flying things. It was one of the latter which nearly did it for me.... The first intimation I had of it was the sudden blotting out of the sunlight from above, and as I glanced quickly up, I saw a most terrific creature swooping down upon me. It must have been fully eighty feet long... with an equal spread of wings... It was coming straight down toward the muzzle of the machine-gun and I let it have it right in the breast; but still it came for me.....

Young traveler and adventurer Bowen Tyler has been marooned with his wife on Caspak, a mysterious and uncharted land far in the southern seas. Here Time's laws have been reversed, and the denizens of a thousand lost ages struggle for supremacy. Amidst these terrors, Bowen Tyler seeks to find and rescue his lovely wife.

By an uncanny stroke of luck, Tyler has contacted the outside world. To his aid came Tom Billings in a powerful seaplane, flying over the unscalable cliffs that wall Caspak from the world. But the tropic jungles of this weird valley are alive with a thousand dangers, and Billings, too, has become lost, leaving his associates, Bradley and the others, behind at the hastily-constructed "Fort Out of Time’s Abyss Dinosaur" to await his return with Bowen Tyler.