Jeff Winston, the protagonist of the novel, dies at the age of 43 again and again and relives his life starting at the age of 18 (for the first replay at least). There are no paradoxes here as Jeff is not going to meet his past self in the other life. The only thing that has been transferred are the memories from his past life. This happens repeatedly, and he remembers all his replayed lives every time he starts anew.
The novel is more nearer to being classified as a fantasy rather than a science fiction one as Ken Grimwood never dwells on the science of time travel. Instead, the story focuses itself on the psychological effects one might have to endure if put through Replays of life.
The whole essence of the book can be summed up by the line said by another Replayer in the book, Pamela who puts it as, "We only make things different, not better."