Sharon Myers unexpectedly turns up at a school reunion party being held in her birthplace of Luckwell, twenty years after leaving the village. She has returned to her hometown to take up the position of Detective Inspector, after recently being promoted. She bumps into her childhood best friend, Helen Andrews, who is heavily pregnant, and attending the party with her current husband, Phillip. Helen is unaware that Sharon and Phillip know each other, and that they had an affair shortly before she discovered she was pregnant. During the party, Sharon notices that Phillip is making eyes at new schoolteacher Lesley Tenant. The following morning, Sharon is given an early wake up call by D.S. Harry Fielding, who informs her that a body has been found in a nearby lake. Sharon arrives at the scene to discover the body is none other than Lesley Tenant. Initial reports suggest that she was stabbed to death, and that her handbag, mobile phone and necklace have all been stolen. Suspicion initially falls on Phillip, but Sharon is quick to dissuade the idea that he could be the killer. Phillip is arrested, but after an initial round of questioning, he is released without charge. Meanwhile, D.C. Marcus Watson discovers that only three necklaces of the kind Lesley was wearing have been bought locally in the past six months, one of which was purchased by a cash-buying customer, who matches Phillip's description. After a series of e-mails are discovered on Lesley's hard drive, signed "Love from P", Harry is determined to haul Phillip in again, but Sharon convinces him to bide his time, and instead look into one of Phillip's work colleagues, Paul, who was in a relationship with Lesley up until six weeks before her death. Meanwhile, after being invited round for supper by Helen, Sharon goes in search of some old childhood photos in Helen's garage, and unexpectedly comes across Lesley's necklace, hidden inside a music box. Sharon confronts Phillip, but he denies murdering Lesley. During their heated confrontation, Sharon's rage turns into lust, and the pair end up having sex. Sharon informs Phillip that she intends to protect him, whatever the cost. The next day, Harry and Marcus interview Paul, and he confirms that he was the one who sent the series of e-mails. Harry, however, informs Sharon that Paul denied all knowledge of the e-mails, and that in her absence, he has applied for a warrant to search Helen and Phillip's house. The search proves fruitless, but in order to disguise her loyalty to Phillip, Sharon orders a search of his workplace. During the search, Harry finds a tube of lipstick hidden behind a cabinet. The lipstick is sent for forensic analysis, and Phillip is re-arrested. However, once again, a lack of evidence leads to his subsequent release. When he is forced to explain his absence to Helen, the unduly stress causes her to collapse, and she is immediately taken to hospital. Later that night, Sharon meets with Phillip, unaware that Harry is watching her every move. Early the next day, Sharon hauls Harry in after discovering he lied about who sent Lesley the illicit e-mails. In order to point suspicion in Harry's direction, she accuses him of holding a flame for Helen, and alleges that his grudge against Phillip is purely personal. Harry, however, fires back with the accusation that Sharon has been protecting Phillip and that he was witness to her meeting with him the previous night. Unwilling to show bias, Sharon questions Phillip once more, but when Phillip questions her about Lesley's necklace - which as far as her colleagues know, is still unaccounted for - Harry realises that Sharon hasn't been telling him the whole truth. Suspecting that Sharon may tamper with the DNA evidence found on the lipstick in Phillip's office, Harry asks the lab to contact him - and him only - with the results. As suspicion gradually begins to fall on Sharon, Phillip approaches Harry and warns him that he believes he is being framed. Harry and Phillip covertly break into Sharon's house, and they find Lesley's missing mobile phone and handbag. Phillip suspects that Sharon is framing him in order to prove her ability to control, and to eventually end his relationship with Helen. Realising that Sharon has gone to pick Helen up from hospital, Harry telephones the hospital, but is told that Helen was discharged twenty minutes previously. Racing across town, they find Sharon and Helen down by the riverbank, engaged in a tussle on a jetty. Phillip suspects that Sharon is trying to kill Helen, just like she killed Lesley. During the tussle however, Sharon falls from the jetty and subsequently drowns, seemingly closing the case. Her death is recorded as accidental by the coroner, and the investigation into Lesley's murder is closed. Helen and Phillip's baby is born, and they subsequently hold a welcoming home party. During the party, Phillip goes to look for some firecrackers, and finds a piece of Lesley's jewellery in the garage. Phillip realises that Sharon is not the killer, and that Helen in fact murdered Lesley after witnessing them having sex on the night of the school reunion.
