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.

Will is settling into the new routines of his life in Grantchester: sermons and services in the church, working the punch bag at the gym, and suffering sniping comments from Leonard in the vicarage.

But then an unexpected phone call summons Will back home, and he asks Geordie for his help: Will's father has got himself into some trouble. It's a chance for Geordie to discover where Will really comes from, and it's not what he expected, taking him into a very unfamiliar – and unfriendly – world.

Will hasn't been home in a while, running from a past he wants to forget. But you can't outrun it forever, and when one vicious beating leads to a grisly murder at a sprawling ancestral home, scandal threatens to engulf the Davenport Family, and life will never be the same for Will again.