Bill Starbrook, an engineer and a devoted family man, discovers a hidden Children’s Room in the university library where his genius son Walt has been checking out books which nobody except himself and Walt can read. In some way that’s hard to explain, that leads to mutants on Earth, an alien invasion, a worry that the mutants are going to take away Walt to save mankind, and (in passing) a requirement that the Children’s Room be moved to a different time.

It’s fair to say that this story’s not about the time travel.

In a post-Hiroshima world, Dr. Dell resigns from a weapons lab to farm, and when Dr. Curtis Johnson visits to persuade him to come back, he finds that Dell’s reasons are linked to time travel.