Detective Inspector Matthew Venn is standing by his kitchen window when he first spots them. Two young girls, facing away from him, seemingly staring towards something in the distance. They are holding hands, and they are alone. Though not a natural with children, Matthew knows he must find out why the girls are here, on a school day, unsupervised. And so he meets Olivia and Imogen, a pair of sisters whose secrets Matthew must uncover if he hopes to get them home.