In the small village of Saveock Water in Cornwall live the Burnett siblings.
Parentless, they keep to themselves and make their own way.
For some reason, though, the older girls keep the younger two under close watch, locking their door at night.
The youngest, Lillian, thinks they're plotting against her.
Lillian's mind is already unstable when she meets Davy Polglaze.
Committed to St Lawrence's Mental Asylum, she plans her escape, but with rapid cultural change and industrialisation, Cornwall may not be a place where she can survive. As Lillian struggles, May is also drawn into her own turmoil.
Their elder sisters, regarded by the villagers as spiritual healers, are far from being benevolent; their use of folk medicine sets a disturbing tone. Loneliness and isolation threaten to engulf them as they try to survive.
When the opportunity arises for them both to leave Saveock Water, it seems that their troubles, imagined or otherwise, might have travelled with them. With Lillian seemingly losing her grip on reality and May subject to her own horrors, it would appear that a return to Saveock Water for both of them is inevitable.
But what awaits the sisters there?In Victorian Cornwall, superstition and madness are close acquaintances.