It's the near-future and Britain is having yet another heatwave. Of course, the government have put in the normal curfews for this kind of weather, and shops are forced to shut again. For Polly, it's the sort of heat that makes her do wild, out-of-character things just to cool down. Like face her fear of deepwater. Essential when she and her brother have been sent to their aunt's eco lake-side house for the summer. But Truthwater Lake is beginning to dry up. As the water level diminishes, a lost village emerges. Swimming over the rooftops at midnight, Polly dives down and is suddenly able to breathe, to hear church bells and bird song. Polly has discovered an underwater gateway - to the past.