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 to help, but are unable to stop her obsession with the woman who died with Greg.

Ellie goes to Milena's workplace under 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.

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.