I. The narrator recounts his longstanding desire to communicate with inhabitants from another world, and discusses the facts that have led some people to think that Venus, Mars, or the Moon might be inhabited. To get from the Earth to the Moon would require a projectile, and would immediately run into the problem of the immense acceleration required. But to get from the Moon to the Earth is not such a formidable problem. Thus the narrator has an interest in meteorites.
II. This brings him into contact with a mineralogist, Edward Daniel Clarke, whose lectures put him on the trail of a large mass of meteoric iron in Siberia. On the way there, he meets up with a strange man in Pekin (modern day Beijing), named Mono, who is also interested in meteorites. The proceed together to Siberia, and in the course of their journey have a discussion about what the sky would look like to someone viewing it from the Moon.
III. Mono reveals that he is in fact from the Moon; he came to Earth looking for his brother, who, being of the daring sort, had first made the attempt to travel from the Moon to the Earth. The narrator wonders if Mono is crazy.
IV. Further questioning by the narrator elicits more explanation from Mono. They agree to go together to Peru in the hope of finding there some evidence of what happened to Mono's brother.
That the story was never finished is one of the misfortunes of philosophy, I think; what Todhunter gives of it is interesting, if somewhat digressive. And you get really nice passages like this, when Mono describes the history of the Moon-men, who live on the side of the Moon facing away from the Earth, as they discover the Earth:
It was an ancestor of my own, named Tisiri, who had the privilege of making this discovery, the greatest event in our lunar history, and I think, the greatest and most striking scientific discovery ever made by any inhabitant of the solar system....You may try to conceive my ancestor's feelings when, climbing the last ridge of the boundary mountains, he saw resting, as it seemed, upon the mountains of the opposite horizon this vast luminous orb: for it so happened that at that time the earth was at full, as you say of the moon: that is, her whole disk was enlightened. Tisiri, in his travels which he wrote and which we possess in our family, gives vent to his feelings in the most rapturous language. 'I saw,' he says, 'that mysterious center which we had long felt, as an invisible power, shaping and controlling the movements of our planet, expanded before my eyes into a vast luminous orb, vying in splendour with the sun and far larger than he. Considering how vast an influence this orb exerts upon our world I was disposed to fall down and worship it, but better thoughts prevailed, and I knelt down and thanked him who made both it and our planet, for having permitted me to see it, first of moon-men, and to disclose its existence to my fellow creatures.'
The story clearly draws on the Plurality of Worlds debate, which was raging at the time throughout the learned world: Are there inhabitants on the other planets in the solar system? The Yes answer had become overwhelming popular, being explicitly advocated in a number of public works and having a certain plausibility due to analogy and certain religious considerations. Whewell had contributed to this debate in 1853 with his work, originally anonymous, The Plurality of Worlds (he added the dialogue in 1854); it became an instant hit, widely published and widely attacked. In that work he essentially rips apart the case made by proponents of the idea of inhabitants on other worlds, systematically showing that it was based on speculation, not evidence and that, indeed, most of the evidence pointed the other way and that the evidence that did not was not sufficient to establish even any probabilities. But he seems to have also been interested in human fascination with the idea of otherworldly inhabitants, which he notes is a recurring idea.