4 stars. With this premise, it's s easy for the story to become too far fetched and even silly. But it's too well worked out, too rationial, even too serious, to be called silly. And that's a huge advantage. 
 It was a bit too long and not thrilling enough, but it's a lovely book as an intermezzo. 
 Better than Kate Atkinson attempt, but not as memorable and engaging as The time traveller's wife. 
 Don't ask me why, but If you liked this I would advice https://www.goodreads.com/book/show/6... and https://www.goodreads.com/book/show/3...