Fast-paced, decent characters if maybe a little sketchy, and a nice explanation of parallel worlds for those who haven't come across the concept. It's only science-fiction because of that one device - you accept that they have it and no one does anything with the concept except use the device. Otherwise a straight thriller: will they find the killer before he strikes again? At first I was perturbed by the scenes from Maddie's POV - I didn't want to read about her death over and over again. But the author doesn't put us through that for nothing. If you slow down and think about it, there's a lot of questions to be answered, but go with the flow and it's a satisfying ending.

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