Following the death of her parents, Rachel Fell (Amanda Abbington) invests her inheritance in Castringham Hall, a derelict Tudor house in East Suffolk. Rachel and her husband Simon (Reece Shearsmith) embark upon a project to renovate and restore the ruined house - once the family home of Rachel’s distant ancestors - to its former glory, and hire a local historian, Mr Crome (John Sessions), to research the Fell family history.

But Crome’s accounts of her relative Sir Matthew Fell’s strange and untimely death, and of the ‘Castringham sickness,’ a disease afflicting the surrounding wildlife, fall on deaf ears. Tensions rise as Rachel antagonises locals with her decision to unearth an unhallowed grave bordering the estate, and as her strained relationship with Simon deteriorates, the couple grow further apart than ever.

Despite Simon’s attempts at reconciliation, Rachel moves into the room where her cursed ancestor, Matthew Fell, was discovered one morning, ‘dead and black’.

The room beside the ancient, dying Ash Tree…