Sidney Chambers attends a talk by the Reverend Nathaniel Todd, a key part of the Civil Rights movement in America. During the event, protestors 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 is called to investigate an apparent break-in at a computer laboratory. When the head of the lab keels over and dies from mercury poisoning, Geordie suspects foul play - particularly when he finds that the Professor appears to have spent the night in the lab with someone other than his wife.
As Will is preparing to be installed as the new Vicar of Grantchester, Geordie comes to him with a request of a religious nature. A young boy on a farm in the Fens has blood on his hands and is struggling to communicate. As Will starts to talk to the boy using sign language, the pair must come together to help the boy's family heal - while Will has demons of his own to wrestle with.
Will, upset by his family tragedy and faced with life-changing choices, seeks distraction in Geordie's world. A strange new case unfolds: the murder of a "Teddy Boy" at a dance hall, which in Geordie's opinion is proof that the country has truly gone to the dogs. The dead boy's pockets reveal a library card from Geordie's daughter, Esme's school, and even more disturbing - a picture of Esme herself. Esme claims ignorance, but Will knows something is up as he's spotted her hiding a love letter.