© ITV/Mark Bourdillon

The body of a woman with a mask on her head is found in the rain-swollen river at Midsomer St Claire by Ava Gould and Zach Lime, who are checking rising water levels for the River Authority. It's Nancy Dewar, a nurse who recently left her builder husband Michael to live with art historian Philip Hamilton. She was struck on the head before being thrown in.


Nelson is struggling to cope with lodging in Kate's messy house. Kate finds his fitness and green tea obsession equally challenging.


Professor Hamilton is restoring a medieval fresco discovered in a crypt at St Claire's Church. Barnaby and Nelson are shocked to see it bears the image of a drowning woman wearing a mask as well as other macabre scenes. Kate's tests reveal Nancy's mask is made of vintage agricultural sackcloth bearing the letters SHC.


Former vicar Arthur Gould claims Nancy was always wild. He has leukaemia and is being nursed by Ava, his daughter. Ava's mother died when she was young and the two are close.


Nelson finds car tracks near the river and dives in to retrieve Nancy's rucksack, containing a work visa and one-way ticket to Australia. Hamilton is cagey about their relationship and tells detectives that Michael assaulted his wife at Christmas. Meanwhile Michael's debt-ridden father Frank goes to Dewar's building yard where Nancy's Fiat car is hidden.


Michael names shopkeeper Ewan Evans as his alibi. Ewan has his own worries, as his son Noah seems interested only in the fresco and not the future of the mini-market. Ewan is terrified when he finds Noah has a book depicting medieval art and images of torture.


Ambitious new vicar Martha Hillcott holds an emergency meeting; she needs money to protect the fresco from flood waters and plans to sell the Goulds' cottage against their will. Frank misses the vote - Nancy's car has broken down and he pushes it down an embankment in his panic. While wiping his prints he discovers documents in the footwell.


Martha has a sealed vote from Nancy which wins her the cottage while Frank returns to the yard, distraught. He hears a noise and is crushed by a digger full of rubble, like a modern form of stoning. Rev Gould says the murders are purges - punishment for sinners. Michael confesses his true alibi - he got drunk after checking into a drying out clinic. Tapes at the yard show Nancy driving off with Zach - a one-time trainee for the priesthood whose brother has convictions for identity theft. Zach admits buying patient records from Nancy.


Barnaby and Nelson find Noah with Hamilton - they are lovers. They claim Nancy never returned that night and Barnaby realises she visited Martha instead to give her her vote in return for cash. Did she collect any other debts that night?


Hamilton opens a letter to Nancy bearing the logo SHC. He's shocked by the contents but that night is shackled to a wheel and impaled. As the rain falls and villagers flock to the church for protection, Rev Gould takes to the pulpit and Ava tries to flood the crypt. In the midst of chaotic scenes, Barnaby unearths a long-held secret that takes him by surprise.