'Please, I need your help. You're the only person I can turn to. The only person who can pretend to be me...'When Alice gets a desperate message from her estranged identical twin sister Cecily, she can't help herself. She has to say yes. Pushing down the memories of the terrible childhood that ultimately tore them apart, she arrives at her sister's house. Cecily has left instructions: how to work the oven, what to cook for supper, what to say to her husband. She says she'll only be gone a week. She just needs to contact a lawyer, sort her head out, and work out how to get away from her marriage safely. Alice barely knows why she's agreed. But as she puts the key in the door, she wonders if - in coming to help her sister - she might be putting herself in the most terrible danger. Because when Cecily stops replying to messages, and the words 'die, die, die' appear scrawled in lipstick on the bathroom mirror, Alice realizes - someone is out to get Cecily. And now they think Alice is her...