Spoilt society girl Cressida Barker-Powell confides to her journal that she plans to murder her unbearable mother-in-law. But when she arrives at the scene, she finds the old woman already dead. Obviously her Hitchcock-Movie-loving best pal, Monica, has carried out the deed for her Taking the murder-switch idea from the movie Strangers on a Train, Cressida decides the only proper way to show her gratitude is by killing off Monica's philandering husband and his bimbo girlfriend. After all, Monica of all people should appreciate the idea of swapping murders? That's what she wants, right? Wrong. Cressida quickly discovers that unfortunately this was not what her friend had in mind, and now Monica is devastated and planning to exact a terrible revenge. Which means their friendship is definitely over. Isn't it?