A lonely widower. A body in a stream. An impossible puzzle.

It's an ordinary Tuesday morning when pensioner Alun Edwards turns up at Llangwelli station. He doesn't want to make a fuss but he's worried about his friend Gareth. Every Monday, they meet up for a game of chess. But Gareth wasn't at home yesterday. He's not answering his phone - and he's left his dog behind. He'd never do that.

Detective Norman searches Gareth's cottage - and finds a woman's photograph, torn to shreds and dumped in the bin.

The next day, the body of an elderly man is discovered in a stream under an old railway bridge. The name on his bus pass is Gareth Jenkins. But that's impossible. According to the pathologist, the body has been in the water at least three or four days. And Gareth was seen walking his dog, just two days ago.

Norman is facing an impossible puzzle. And what is the secret behind the woman in the photograph . . . ?