In the heady swelter of a suburban summer, the Elm Hill lido opens. For teacher Natalie Steele, the school holidays are always spent with her husband Ed and daughter Molly. But not this year. Despite Molly's extreme phobia of water, Natalie is drawn to the lido and its dazzling social scene, led by glamorous Lara Channing. Soon she's spending long days with Lara at the pool and intimate evenings at her home. Real life begins to feel very far away. But is the new friendship everything it seems? Or, without realising, has Natalie been swept dangerously out of her depth?