This is a science fiction novel about Twin Earth, but it reads like a detective novel.
The time background, environment and main story line all revolve around Twin Earth.
The time and historical process on the two earths are almost the same, so at the beginning of the story, the protagonist "I" was not aware of the existence of another earth. I was accustomed to the two moons in the sky, and the investigation of mysterious events gradually revealed the truth about the two earths...