This was a time travel story and what happens in the present and the future to three people, Marina, Finn and James.
I kept thinking I should be more bothered by the solution to the conflict which was given at the start of the story. I am a little bugged that I was not really that bothered.
My biggest problem came from the characters when they had flashes of memory. There was little indication when the memory was over. The paragraph would just continue and you were supposed to see that the memory had finished. A couple of times I did get confused. After a while I got used to it and it didn't throw me for as much of a loop.
There were some stories hinted at without any real expansion. One was a characters death in the future, but there was no detail of why and little detail of how.
I did not like Marina. She was a snob and a snot for a good portion of the book. James had little depth in either time. He let his grief warp him until he had become a monster, but that was his most defining character trait. I really liked Finn, he was a good guy in both times and his experiences didn't change that. Em's personality shift was well done. Her experiences affected her and the damage was plain to see without being overbearing.
The science and paradox were both addressed and realistically enough not to be distracting from where the author was taking the reader.
The resolution made the rest of the story worthwhile. I enjoyed Em's actions and reactions. It showed that despite the problems Em had faced her character was not completely different from when she had been Marina.