It's 1956 and with Amanda long gone, Sidney's life now revolves around the church services and the interminable meetings of the Parochial Council and their plans for a village fete. He yearns for some excitement, and Geordie has noticed that Sidney's devil-may-care attitude feels increasingly like a death wish.

Sidney Chambers attends a talk by the Reverend Nathaniel Todd, a key part of the Civil Rights movement in America. During the event, protesters disrupt Todd's speech and release fireworks, causing the audience to panic and leading to a crush as dozens of people try to escape through a locked door. In the chaos, a man is stabbed and killed by an unseen assailant. The murder sees racial tensions spike and Geordie is called in to investigate.

Geordie suspects foul play when computer laboratory head Professor Simms keels over and dies from mercury poisoning. It seems the deceased recently spent the night in the lab with someone other than his wife, possibly a fellow educated man. Geordie seeks advice from Leonard, and the pair start to uncover the surprisingly messy love lives of Simms' colleagues. Elsewhere, Cathy is brutally tricked, Will returns to Grantchester and Mrs C makes a shocking discovery.