As a steam train races through the Devonshire countryside, Humphrey and Esther join the Shipton Abbott Players for a murder mystery rehearsal. Though Humphrey is only playing a detective, things turn from amateur to professional when the actor playing the victim is found dead with a real knife in his back.

Life begins to imitate art as the pretend suspects become all too real, and the steam train is scoured for genuine clues. With the victim barely known to his fellow performers, and the whole group under suspicion, the team sets out to unravel a dark past that reveals hidden connections between the actors.

Meanwhile, love is in the air as Humphrey and Martha enjoy date nights and Anne throws herself into the world of online dating. But has she bitten off more than she can chew? And it's all change for Humphrey and Martha as they start a new chapter in their lives, embarking on the process of becoming foster parents.

A fishing trawler heads out to sea with three brothers and their ailing stepmother aboard. When it returns to Shipton Abbott harbour several hours later, only the three fishermen remain. A note found on deck suggests the woman took her own life. As her estranged daughter comes to town to help make funeral arrangements, the simple case grows more complex.

Meanwhile, it's the day of the house visit, but it's no plain sailing for Humphrey and Martha. Will the houseboat hit the mark or be an obstacle in their path to fostering?