Schoolteacher Ellie Manning hears the devastating news that her husband Greg has been killed in a car accident, and her grief is compounded when police tell her that Greg did not die alone - another woman, Milena Livingstone, was in the car with him. Faced with the shattering knowledge that Greg was having an affair, Ellie is determined to discover everything she can about Milena. Ellie's best friends Gwen, Fergus and Joe rally round to help, but are unable to stop her obsession with the woman who died with Greg.

Ellie goes to Milena's workplace using a fake name and offers to help sort through Milena's paperwork on the pretence of being helpful - but really looks for clues as how Milena knew Greg. Milena's business partner Frances Shaw confides in Ellie that her own marriage is over as she had fallen for Milena's lover. When Ellie asks Frances who her lover was, they are interrupted but agree to meet later at the office to talk. However, when Ellie returns, she realises that she is too late.

After confessing her true identity to an angry Johnny,Ellie unsuccessfully tries to persuade a police inspector that Frances's and Greg's deaths were linked. Drunk and abusive,Frances's husband David turns up and admits to an affair with Milena to spite his wife's infidelity but has an alibi for her murder. Then Hugo visits Ellie with a letter that shows that Milena and Greg only met once,on a professional basis,and,after visiting their client Ellie is aghast to discover that somebody she trusted murdered three people. Obtaining a result,however,almost costs her her own lif