Out of Time's Abyss is a direct sequel to The Land That Time Forgot and The People That Time Forgot, continuing the lost world saga begun in the earlier stories. It connects the previous two installments, bringing in characters introduced in each. Burroughs completes the revelation of his lost world's unique biological system, only hinted at in the previous installment, in which the slow progress of evolution in the world outside is recapitulated as a matter of individual metamorphosis. This system forms a thematic element serving to unite the three otherwise rather loosely linked Caspak stories.
The book begins with Bradley, who had left Fort Dinosaur on an expedition in the first novel and never returned. Bradley and his party are attempting to return to Fort Dinosaur. Along the way, they encounter a creature which appears to be a flying dead man. Some of the members of the party consider it to be a ghost or banshee. Tippet is convinced that he is soon to die, and the next day he is killed by a Tyrannosaurus. The ghost-like creature is seen again, and James is killed by a Smilodon. Bradley disappears during the night, and the remaining members of the party make it safely to Fort Dinosaur.
Bradley had been captured by the ghost-like creature, which is soon revealed to be a naturally winged human being, belonging to a subgroup of humanity known as the Wieroo. The Wieroo takes Bradley to the island of Oo-oh, set in Caspak's inland sea. It attempts to keep Bradley in a prison, but he escapes through a secret passage. He meets Co-Tan, a member of the highest human race of mainland Caspak, the Galu, fully human and of a neolithic cultural level. They enter the chamber of the Wieroo king, a huge member of the race, and Bradley kills the creature with its own sword.
Co-Tan and Bradley escape the city of the Wieroo and live for several months on the forested coast of Oo-oh. Finally, though, they are discovered by Wieroo. They succeed in capturing two of the Wieroo and forcing them to fly to the mainland, one bearing each of the humans.