After holidaying in France with a group of old university friends, Isobel and Eric discover that Ben and Maggie's baby girl has contracted the measles virus and is now permanently deaf. Isobel knows that her decision, taken years ago, not to have her own children vaccinated is to blame for Iris's deafness. And Ben knows this too. To make matters worse, Isobel is the woman he fell in love with in his twenties - the woman who married his best friend. As he and Maggie start legal proceedings against his ex-lover and his best friend, Isobel's world begins to unravel.