Again time-travel. The organization called Eternity controls time. They alter it by making subtle changing in the past or future, for the good of mankind as they declare. It’s named Reality Change.
In one of the time travels Harlan meets a girl and after a while, he falls in love with her. The Eternity is against this as they say it can endanger time in a bad way. But the girl wasn’t an ordinary one. She has come, you won’t believe it, from beyond 100.000th century.
She says that not our deeds but the existence of Eternity itself is deleterious for future and humans. She proves that as the Eternity consumes a lot of energy which humanity would have used to reach stars if it didn't exist, it must be destroyed.