It's summer in Adders Fork. The sun is out, the sky is blue and things are going swimmingly for Rosie Strange, thank you very much. The Essex Witch Museum has been relaunched with a new Ursula Cadence wing and picnic grounds. Takings are up and Rosie and curator Sam Stone are getting on very well indeed. Then developers roll into the sleepy village to widen the road. When the centuries-old Blackly Be boulder, said to mark the grave of a notorious witch but now in the carpark of the Seven Stars, is moved, all hell breaks out. Within hours a slew of peculiar phenomena descends: trucks break down, blood seeps through walls, headless monks and spectral ladies are sighted in the dead of night, and the church clock strikes thirteen. And have gold coins, supposedly the lost treasure of Mockton Abbey, been discovered?

When a severed head is discovered atop the boulder, the locals can take no more and storm the Museum to demand someone take action. Can Rosie and Sam unravel the mystery? And what of the ancient treasure that could drastically change someone's fortunes and offer a motive for murder?