© ITV/Mark Bourdillon

Coma victim Harriet Farmer, who was attacked on the night she planned to elope, wakes up. Her father David hears the news in the middle of a chess tournament. Harriet's boyfriend Finn Robson has been missing since the couple tried to run away together a year before.


At the chess tournament, hosted by hotelier Arthur Potts, Olivia Carr is putting pressure on her son Jamie to win. The 13-year-old has already beaten Finn's father Alan, a software boss and fitness fanatic, who made millions from a computer game, 'Chess King'.


Hotel owner Arthur berates his wife Caroline for spending their meagre income on charity work, while hoping to win backing for his business from writer and chess society president Edward Stannington. Later that day, Stannington is found dead in a lake with an ice axe in his back. His damaged car is nearby and Kate finds a clue in his mouth - a chess notation known as the 'Sicilian Defence'.


Olivia admits she met Stannington and rammed his car when he refused to help Jamie get to the top in chess - divulging that he's the boy's father. David Farmer begs Caroline to come to supper, although Arthur has also booked to take her out for an anniversary meal. Meanwhile Jamie is being scared by loud noises at his house, when he is alone.


Jones finds that Stannington was digging into Alan's chess software company Black Knight and received a letter from a disgruntled ex-coder on the day he died. Stannington's aunt Vivian wants to celebrate her inheritance with Arthur - but he's only interested in business.


Caroline angrily leaves Arthur at the restaurant while Farmer prepares supper for her. But when he goes out to fetch a gift he's strangled. The killer leaves another chess code. Arthur gives Caroline an alibi, but - after the death of an elderly resident - Vivian tells Jones she suspects Caroline is poisoning the old people with her meals on wheels.


A lip-reader reveals that Finn's mother Wendy knew that her son and Harriet planned to elope. Alan followed them that night and now he's gone suddenly to London. Barnaby suspects his chess company has secrets. Then Jones finds Arthur with a cat in his freezer.


Harriet comes home from hospital but Caroline barges in, attacks Harriet's neurologist Laura Parr and drags Harriet away. Under questioning, Arthur reveals that Caroline may have unwittingly poisoned the villagers with food from the hotel. Meanwhile Caroline is forcing Harriet through the woods, pleading for information about Finn. The police find them and it's apparent that Caroline is obsessed.


Alan returns to Midsomer tight-lipped. Then he recognises the chess notation in the incident room - it was written by another programmer, Jason Winters, who committed suicide soon after beating

 a world champion at chess. But why is the killer using his code now?


Realising Finn's disappearance is key, Barnaby takes Harriet back to the woods. She calls out Jamie's name - but he's not the only one involved. As the truth about Jason Winters is uncovered, danger lurks at the hospital. Jones makes an astonishing discovery but the killer is one move ahead. Can Barnaby's psychological skills save the day?