'I think you might be my father . . .' Zoe Barry walks into Professor David Connelly's office and tentatively says these words. The lives of his family - particularly his wife Caroline - are turned upside down by the arrival of this stranger. A cuckoo in their nest Zoe soon entangles herself in their world but her stories aren't quite adding up and Caroline isn't sure she can be trusted. Is Zoe who she claims she is, or someone with a far darker agenda? A daughter, a sister, a stranger . . . By letting Zoe in, David and Caroline aren't the only ones who are vulnerable. They're risking the lives of the most precious thing in the world - the lives of their children . . . . . . a killer?