Reports begin coming in to the police that burglars are striking all over Ape City. Valuable items are being stolen from the Museum, Ape Archives, the Library and from ape homes. A first edition William Apespeare book was taken from Zira and Cornelius' home. Into the night, hooded figures sneak to the cemetery to the Tomb of the Unknown Ape where all the stolen items are hidden. In reality, Urko and his gorillas are disguised as Underdwellers in hopes of framing them and convincing Dr. Zaius to declare war on the below world. Zira and Cornelius travel to New Valley to meet with Jeff, Bill, and Judy. Suddenly, Krador appears and tells them the Underdwellers are innocent of the thefts. He asks them to come to their underground city and bring the laser.
Meanwhile, Zaius and the war council hear testimonies of witnesses against the Underdwellers. Unknown to the council, the witnesses are lying as instructed by Urko. Urko assures the council that he can win the war by blowing up the tunnel entrance to the below world. Krador, Bill, and Jeff reach the end of the tunnel underground and Jeff cuts through the wall with the laser. The tunnel connects with the tomb where all the stolen items are hidden. Bill plans to have Cornelius tell Zaius the real truth the next day, but Krador warns that by then Urko will have detonated the tunnel entrance, flooding the world below and causing Ape City to collapse. Krador projects Bill's image into Zira and Cornelius' bedroom where he tells them to take Zaius to the tomb immediately or it will be too late. Krador hypnotizes the guard to awake when Zaius speaks to him and tell the whole truth of Urko's twisted plan . After seeing the stolen goods, Zaius tries to radio Urko to halt the attack, but the general was warned that Zaius knows his plan and he disconnects the radio. Zaius and Cornelius race to the dock to stop Urko personally from sending a barge of explosives into the tunnel entrance. While the Underdwellers hold a final service in the great cathedral with Judy, Krador mind transports Bill and Jeff to the tunnel's entrance. Jeff cuts through the wall and they jump onto the barge. They manage to send the barge back toward the dock where it detonates. Furious, Zaius orders Urko and his men to clean up the damage, personally return the stolen items, and Urko must apologize to the city on TV. To teach Urko a lesson, the senate suspends him from duty for three months without pay.