Subsequent editions have used a variety of titles, and variously presented the sequence as a single work, a duology, or a trilogy.
In "The Moon Maid" (May-June 1923), a spaceship from Earth captained by the hero, Julian, is forced to land on the Moon after the engines are sabotaged by his resentful second-in-command, Orthis. The Earthmen discover the Moon to be hollow and inhabited by several warring cultures, including the Laythe and the Kalkars. After many adventures, during which Julian meets and falls in love with the Moon princess Nah-ee-lah, they are able to repair the ship and return to Earth, taking Nah-ee-lah with them and leaving Orthis behind.
In "The Moon Men" (February-March 1925), Orthis gathers a force of hostile Kalkars and leads an invasion of Earth. Julian confronts Orthis, and both are killed, whereupon the story skips forward a hundred years to when Julian's descendant, also named Julian, becomes involved in the resistance against the Kalkar overlords.
In "The Red Hawk" (September 1925), the story skips forward again several centuries to show the final overthrow of the Kalkars by the tribes of humanity, featuring an alliance between the Tribe of Julian and the Tribe of Or-tis.
This series contains examples of:
- All Hail the Great God Mickey!: After Earth is conquered by the Kalkars, the American flag becomes an object of worship to the rebellious underground.
- Call-Forward: "The Moon Maid" contains a lot of foreshadowing for "The Moon Men"; Burroughs actually wrote the latter first, but was unable to find a market for it, so he wrote the former as a more typical Burroughs adventure story in order to create demand for the sequel.
- Canon Welding: The technology for the Moon mission is Barsoomian in origin.
- Disintegrator Ray: Orthis invents one which he uses during his invasion of Earth.
- Everything's Better with Princesses: Nah-ee-lah of the Moon, the maid of the title, is Princess of Laythe.
- Exty Years from Now: A near-miss — "The Moon Maid" was published in 1923, and the adventure begins in 2024. Perhaps Burroughs was unduly pessimistic about how long it would take to get into print.
- Green-Eyed Monster: Orthis starts out with a grudge against Julian because Julian was made Captain of the space exploration mission even though it was Orthis's technical genius that made it possible, and gets worse as the story progresses, to the point that he spends decades plotting the Kalkar invasion of Earth primarily as a way of getting revenge on Julian.
- Green-Skinned Space Babe: Nah-ee-Lah, the title character, is a beautiful black-haired, pale-skinned Moon princess who falls for the protagonist Julian of Earth.
- Happy Ending Override: If the Canon Welding is taken to its logical extreme, then every setting related to Earth (except maybe Pellucidar) will be destroyed when the leading faction from the Moon will eventually conquer Earth. Although it gets better eventually, the cost is the loss of most modern governments, some loss of historical records, and a slow climb back to previous technology.
- Hollow World: The Moon is hollow and inhabited.
- Lunarians: The Moon has sapient inhabitants.
- Multicultural Alien Planet: The Moon is inhabited by a variety of cultures, including the warlike Kalkars.
- No Name Given: Julian, the hero, makes a point of not telling the narrator of the Framing Device his full name, because he doesn't want to be easily identified after the story gets around.
- One World Order: Following a series of world-wide conflicts following on from the Great War of 1914, Earth is united under "the Anglo-Saxon race" and governed jointly by the United States and the United Kingdom. This is broken when the Earth is invaded by the lunar people.
- Past-Life Memories: The premise of the Framing Device is that Burrough's Author Avatar meets one of the incarnations of Julian, who remembers his other incarnations and tells their stories. In a twist, it's a 20th-century incarnation recalling the adventures of Julians from the 21st century and later, so either reincarnation doesn't necessarily happen in chronological order or he can also remember future lives.
- Julian, the hero of "The Moon Maid", is reincarnated as his descendant, the hero of "The Moon Men", and again as his descendant, the hero of "The Red Hawk" — and then as his ancestor, the 20th-century Julian who tells the whole story to Burroughs' Author Avatar.
- There's a hint in "The Moon Men" that Orthis is also being reincarnated, and also has some memory of his prior incarnations; during their final confrontation he rants that Julian has always "robbed me of the fruits of my efforts" — "all our lives".
- Sudden Sequel Death Syndrome: The hero of "The Moon Maid" is summarily killed off in the opening chapter of "The Moon Men". (To be fair, he does get 25 happy years in between, we just don't get to hear about them because the narrator is skipping ahead to the next interesting bit.)
- Taking You with Me: Orthis, in his final confrontation with Julian at the beginning of "The Moon Men".
- Vichy Earth: Earth after it is conquered by the Kalkars.