Em is in a cell next to Finn, a boy she cares about, but hasn't seen since they were locked up. The Doctor has been torturing them to get a vital piece of information. And in a hidden place in her cell, there's a piece of paper from a future self(or is that past?): "You have to kill him." The "him" is the boy she once loved, when she was Marina, rich and spoiled, living next door to an even wealthier family with two sons, the brilliant young politician-in-waiting and his shy, geeky but gorgeous younger brother. He invented a time machine and the only way to prevent dreadful things happening is to travel into the past with it and stop it being invented by killing him. Of course, this means that she and Finn, too, will cease to exist...
Time travel novels are great fun, even when they're meant to be serious. You always wonder how the next author will deal with all the paradoxes time travel would cause. And this one has thought carefully about it and worked on the consequences; in the context of this novel, at least, she convinces me. She has also played with all the cliched tropes - eg you mustn't meet your past or future self or the universe will explode or some such, and poked her tongue out at them, in the middle of a dramatic scene. Even J.K. Rowling had Harry and Hermione warned that they must not be seen. It worked in that book, mind.
One cliche she does hang on to is the one where the heroine has a choice of two gorgeous boys, but in this case, the reader knows from the beginning which one she will end up with, just not how.
What I particularly liked, as a fan of old-style SF, is that the mad scientist of this genre is given a background, a reason for turning mad and a time when he was a teenage boy and had family and friends who loved him. It's a nice touch.
If you've read all those enthusiastic blurbs saying that this is for fans of The Hunger Games, forget it; it's not remotely like that book, and I have yet to find a book that is. Those blurbs just cash in on the fame of the other book and don't do justice to either. It's a bit like comparing every fat fantasy saga to Tolkien's LOTR when there's nothing quite like it.
But this is an enjoyable book and well worth reading.