It is the far future of 1967 and a great war is finally over. A man on an airship gets into a conversation with his fellow passenger. This passenger is aware of past and future reincarnations of himself. He tells the story of his future incarnation Julian the fifth.
The year is 2024 and Earth has launched a ship to reach Mars. Unfortunately the inventory of the ship Orthis is upset that he is not in command. While drunk he damages the ship and it is forced to land on the Moon. It descends through a crater and into an inhabited world inside the Moon.
Julian and Orthis are captured by the Vargas, a centaur race of people. While captive Julian is introduced to another captive Nah-ee-lah. She is a princess in the last free city in the Moon. They escape to her city and defeat an attempted coup against her father. Just at the moment of victory the city is attacked by the Kalkars. Orthis has armed the Kalkars with modern cannons and they destroy the city. Julian and Nah-ee-lah find the ship repaired and escape to Earth.
This is the first of three stories about the moon men. Its another fascinating world that Burroughs are created. It is the next two stories that get really interesting.