Martha has a comfortable, satisfying life with her partner, Gabe, a gentle American academic.
Her two daughters, from a previous relationship, are now both adult. Jo, ambitious and independent, is pursuing a career as a barrister. Jo's younger sister is very different. Shelley, just 20, sweet natured, trusting and innocent, still lives at home and works at a local animal clinic.
They are a normal, happy family - and then Shelley meets Sam on the street and everything changes.
Martha struggles to accept Shelley's choice of boyfriend as, with increasing anger, she witnesses the erosion of all Shelley's values under Sam's influence.
When Martha's efforts to persuade Shelley to give Sam up fail, she decides on a more direct approach. The consequences of her actions are devastating for everyone and change the course of Martha's life forever.
"After I'd gone up to bed that night, leaving Gabe amidst the carnage I had created in the living room, I formulated my plan. But it was pure chance that I met Sam on Putney Bridge some three weeks later and I went ahead with it."