"Down to Earth" by Harry Harrison is a science fiction novel written in the early 1960s. The story explores themes of space exploration and alternate realities, following astronauts who unexpectedly find themselves in a drastically altered world upon returning from a moon mission. The narrative highlights the consequences of historical events and the nature of reality we perceive. The plot centers around Major Gino Lombardi and Colonel Danton Coye, who embark on a lunar expedition. Following a tragic accident that results in the death of a fellow astronaut, the duo returns to Earth only to discover that the world has drastically changed. They encounter a Nazi regime that has taken control of the United States in a reality shaped by historical divergences. As they grapple with their predicament, they seek to find a way back to their original reality, ultimately aided by Albert Einstein, who theorizes a method to return them. Their story is one of survival, exploration, and the impact of choices made throughout history. (This is an automatically generated summary.)