Urko and his army are searching the desert for the entrance to the Underdwellers caverns. Suddenly, Urko spots Bill. He radios Dr. Zaius with the news. Bill plans to decoy the army so that Jeff can slip away. Huge rock pinnacles abruptly jut up from the ground, a hole opens up and a flash of energy shoots out. As the doorway closes, the two men crawl in and ride a conveyor down a dark shaft. Urko feels that the Underdwellers are responsible and proceed to find their caverns. Inside, Bill and Jeff find a giant dish serving as part of an elaborate power system . They know the apes wouldn't have had the technology to build it. They are shocked to find what remains of the New York Public Library among the rubble; this upside-down world is Earth! They pass by Wall Street and Times Square. Zaius tells Zira and Cornelius that Urko has located Blue Eyes. A soldier enters with the news that Blue Eyes has escaped and is suspected to be with the Underdwellers. Zira and Cornelius secretly wonder if they were right in helping Bill escape.
Elsewhere, Bill and Jeff enter a huge cavern above a giant power complex operated by hooded and cloaked figures. An alarm rings and the figures begin chanting "Oosa" (USA) over and over. Bill and Jeff discover that the whole complex seems to be wired to a central, throne-like chair. They follow the crowd into another room and spot Judy, also in a hooded robe, standing at the front of the room. Next to her is a tree, a symbol of life on the surface. They call to her, but she only stares into space. Suddenly, a hooded figure shoots a ray out of his eyes and stuns Bill and Jeff. They are awakened in a cell by Krador, who introduces himself as the leader of the Underdwellers, descendants of the humans who sought safely underground during the holocaust long ago. Krador believes that prophecy has foretold of Judy, or USA, helping the Underdwellers return to the surface. Later, Jeff gives Judy a ring which revives her memory. She kills the power on the force field holding the men in their cell and meets them in the corridor. They start traveling on a hand rail car, barely escaping the beams of several Underdwellers. They encounter many illusions sent to stop them, but manage to reach the surface opening. Judy can still feel Krador's control over her mind and just before they emerge onto the surface, Judy disappears and returns to Krador. Bill and Jeff crawl up to the surface and vow to return someday for Judy. Meanwhile, Urko and his soldiers continue to comb the area for Bill and the Underdwellers' caverns.