It's 2032 and Britain is facing yet another heatwave. Temperatures are breaking all records, and now government curfews are in place to ensure people stay indoors out of the sun. But it's also the school holidays and Polly Carraway is fed up.

When a late night beach swim goes horribly wrong she's relieved to be sent away, with her troubled brother Joel, to stay with their aunt, whose house is on the banks of a reservoir.

On arriving Polly's shocked to see Truthwater Lake is drying up. As the water level drops, a lost village emerges. And even here, in the countryside, it's still too hot to sleep. Swimming in the lake at midnight, Polly dives down and is suddenly able to breathe, to touch rooftops and see gravestones and hear bird song ...

Has Polly found a gateway into the past?