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.
Some emergency has come up. I don’t know what, exactly. They’ve got to move the
Children’s Room to some other age right away—something about picking up an important
mutant who is about to be destroyed in some future time.