Cohn surfaces from a deep-sea dive to find that the world has been destroyed by nuclear war. His boat is still there, though all the crew members are gone. He hears the voice of God say that He is creating a second flood to wipe out the humans who have once again disappointed him. Yet Cohn is not another Noah, merely someone overlooked. He waits for his death or the ebbing of the flood, whichever comes first.

He discovers that a highly trained chimpanzee, Buz, has also survived and lives on board. Soon, Buz and Cohn sight a tropical island. Crusoe-like, they set up a home on the island, which apparently has no animal life. The two survive on fruits and rice. Cohn discovers that Buz’s former owner, an eccentric German scientist, implanted a voice box in the chimp’s throat. Cohn connects the wires that jut from his throat and the chimp begins to speak fluid English. Other animal life begins to appear on the island, all of it simian. A gorilla shows up first, a troop of chimps follows. Buz teaches them English.

Cohn imagines that chimpanzees are meant to replace man as the dominant creature on earth and he tries to teach them to avoid the mistakes of man. But the chimps have their own agenda. The dominant male, Esau, tries to "rape" the female when she first comes into heat and threatens all the other chimps. When Cohn mates with the female, producing a human/chimp child, the jealousy of the other chimps becomes a destructive force. Baboons appear by the shore and the young chimps hunt, kill, and eat the baboons, calling them dirty, stupid creatures. When Cohn complains, they steal his child, finally killing her. They then force Cohn to carry wood up the mountain. They slit his throat and throw him onto the fire, built from the wood he carried.