Ned woke up to the face of Lord John staring down on him and he is overjoyed by the news the others are not dead. He is taken aback by Lord John's appearance and behavior though, because he is wearing torn clothing and his face is scratched and has dried blood on it. Lord John is imploring Ned to get up, gather guns, ammunition, and food as quickly as possible. He also tells Ned to remain silent and follow him as quickly as possible.

The reason for his appearance and his haste, is because he and the others were caught by ape-men while Ned was on his walk the night before. These creatures, who are a mixture of human and ape, have their own language and social structure. They have been watching the expedition since they arrived at their camp and the night before descended on them from the gingko tree. They took the men and tied them up as they lay on the ground.

Challenger managed to rise to his feet and loudly voiced his objections to how he and the others were being treated. It is then the chief of the ape-men noticed he and Challenger were very similar in looks, because of this he decided to treat Challenger as his friend. Summerlee and Lord John did not have such an advantage, but Challenger was able to help them in a limited way.

After the men had been marched three to four miles to the ape-men's village, Challenger was treated very well while Summerlee and Lord John were tied up beneath a tree. Challenger convinced the chief to allow him to feed them some fruit and while he did this he also loosened their bindings.

The ape-men occupy one side of the plateau and humans hold the other side, because of this the two sides are constantly at war. While the three men were in the ape-man village, they witnessed the treatment of some humans who were caught by the ape-men. The ape-men killed two of the humans immediately, and had four of them jump to their death on the sharp bamboo, where the three men had found the skeleton earlier. Lord John is afraid the ape-men have every intention of having Professor Summerlee jump also.

Lord John escaped that morning and made a beeline to the camp in hopes of finding something to help their plight. He is very happy to have the help of Ned. The two of them evade the search parties, which are looking for Ned and Lord John, and arrive back at the village in time to kill the chief, just as he is about to order the execution of Summerlee. The men shoot many of the ape-men and return to their camp.

Just as they are about to celebrate their escape from death, the remaining human's show up and want the men to protect them. The four men are now trying to return the humans to their cave village. Professor Challenger has told Ned he would not be happy if Ned mentioned in his articles Challenger's resemblance to the ape-man chief. Ned agrees to Challengers request.

The humans who are called the Accala, are a tribe of Indians, who live in the caves Ned had seen the previous night. There is a young man who the other tribe members are very deferential to, his name is Maretas. The Indians call the ape-men, Doda and they are afraid of them, but at the same time they want to eradicate them from the plateau.

Ned is caught by a Doda, while he is looking for a missing Accala. The Doda tries to break his neck, but Lord John shoots at him and Ned is rescued.

The men and the three remaining Accala reach Lake Gladys, where they are met by a large contingent of tribesmen in canoes; leading this group is Maretas' father, the chief of the Accala. The men are a rescue party in search of the chief's son and his other tribesmen. The men decide, after an impassioned speech by Maretas, to enter into a final battle with the Doda. They ask Ned and the others to join them in this last battle, which the men agree to do. They meet with resistance as they approach the Doda village, but the Accala, with the help of Challenger and the others are successful. The Doda males are all put to death and the females and children are made slaves to the Accala.

With this war over, Challenger agrees to focus all his attention on finding a way off the plateau. He has found an inflammable gas, which is lighter than air, near the lake they camped at before setting out to destroy the Doda. He tells Ned he can use this gas, but Ned does not know how he will use it. At the same place Lord John has found some more blue clay, which seems to make him happy. While at the lake, Challenger and Summerlee observed some fresh-water plesiosaurus, which gave them a thrill beyond words.

Ned and Lord John rescue Professors Challenger and Summerlee from the ape-men and return to their camp. They are then drawn into a battle between the Indian tribe called the Accala and the ape-men, who are called the Doda. The men realize, after the battle, it is time to spend all their energy on finding a way off the plateau.