How can you tell in a TV thriller that someone’s life is about to go horribly wrong? Answer: when they are shown to be living a smug, cosy middle-class existence. And in the first episode of Cheat , the life of university lecturer Leah Dale (Katherine Kelly) seems to be just that: on the verge of a permanent job; trying for a baby; living in a comfortable book-lined home in leafy Cambridge; riding to work on her bicycle on sunny days… What could go wrong?Well, as this is an old-style psychological thriller, seemingly cut from a well-worn ITV template, everything. All that Leah holds dear is about to be torn apart.And from the moment we see student Rose Vaughan (Molly Windsor), we know she will do the tearing. It is easy to tell, simply from the way she looks at the end of each scene – anyone who has such mastery of the knowing half-smirk is going to be bad. It is the TV trope of the quietly evil.From extras.Rose submits a dissertation that is excellent – so excellent it bears little resemblance to her other work. Leah questions her about it, Rose gets stroppy and fails to provide any evidence of her research, so Leah fails her on suspicion of cheating.While Rose seethes, it emerges that Leah’s life is not quite as good as it appears: the attempts to become pregnant haven’t worked; she’s having sexual fantasies about her boss at college; and she should be taking medication but has stopped to get pregnant (we don’t know what the tablets are for – the bottle says Dosatriptyline, which, according to Google, doesn’t exist – but I would put money on it being for a mental condition. Really, it seems that predictable.)Having lit the drama’s blue touchpaper by failing Rose, Leah signally fails to stand back, instead spitefully humiliating the student during a lecture.