When Sadie Wise strikes up a seemingly innocent conversation with a stranger on a train, the only thing on her mind is finding her husband Eddie and making him sign their divorce papers. She tells Mona that Eddie has been avoiding her for years but now Sadie knows where he is, she can finally be free from him. In Sadie's mind, it's a throwaway moment. In Mona's mind, Sadie is asking Mona to do something very dangerous for her.

That one chance encounter sets off a chain of events involving murder, deception and danger and Sadie soon realises that Mona has taken their meeting very seriously. Because now Mona is everywhere and she won't leave Sadie alone until she's fulfilled her side of their 'deal' . . .