Dolores is a private gardener specialized in sizeable estates. On a day when her employer is away, her curiosity gets the best of her and she finds herself wandering inside their home.

Beneath the surface of an English river, something ancient is awake. When wild swimmers begin vanishing without trace, trainee journalist Deji decides to investigate. But the further downstream she goes, the more the old stories resurface. And when the river turns red, it's time to start swimming.