In the past: a man crawls desperately through a claustrophobic escape tunnel beneath a POW camp in the Cambridgeshire Fens. Above, a shadow passes across the moon, while ahead only death awaits him.

In the present: Philip Dryden is reporting on an archaeological dig at the old POW camp when a body is uncovered. But there is something odd: the man appears to have been shot in the head, and the position indicates that he was trying to get into the camp, not escape it.

It's a puzzle which excites Dryden far more than the archaeologists or the police.

That is, until a second, more recent, body is discovered ...