Dr Hudson tells his friends about a murder that took place over thirty years ago when he was enjoying a leisurely cruise on the Ormskirk Canal. He was photographing the Roundhead Arms, enjoying the canal's tranquil setting and agreeable air, when purely by chance he photographed Simon and Martha Dunn coming out the inn. A few days later Martha is found with her throat cut and Dr Hudson asks the group if they can solve this long forgotten murder mystery. A group which includes Inspector McGowan.