Mia Cosgrove is a high-flying psychotherapist with a thriving practice, but when she receives a desperate phone call from her oldest friend, Lysette, she puts her London life on hold to rush to her side. A friend of Lysette's, Sarah, has plunged to her death from the top of a multi-storey car park, a text message on her phone next to her, simply saying 'I'm sorry' with a single X, left unsent and unaddressed. At first the police are convinced it's a suicide, but when another death rocks the rural community Mia is asked to help the ramped-up investigation. Why are the close-knit group of mums who surrounded Sarah so reluctant to share what they knew about their beautiful, troubled friend? And how high a price will Mia pay for her determination to unearth the truth and discover what really happened?