For a series set in our future, Star Trek revisits the past constantly. Kirk and Spock battle Nazis, Roman gladiators, and witness the Great Depression. When they're not doubling back on their own earlier timelines, the crew uses the holodeck to spend time in the American Old West or Victorian England. Alien races have their own complex and fascinating histories, too.

The Star Trek universe is a sci-fi imagining of a future world that is rooted...