France in the summer of 1936. The village of Villy is in a state of contentment, tinged only slightly with unease. A kilometre away, construction is underway on a large underground fortification, part of the Maginot Line project which has seen the building of a series of defences against future invasion.

A young artist has arrived in the village to paint the landscape. Her name is Nyssa and she has taken a room in the local inn, run in its owner’s absence by a young Australian woman called Tegan. But she's not the only newcomer. A detective called the Doctor has just got in from Paris. And he has quite a mystery to solve...