Sidney and Geordie investigate when a man is embraced during a talk by a Civil Rights activist, sparking racial tensions.
With Violet gone, Sidney loses himself in drink. After a heavy whiskey session leaves him without his jacket - and little memory of the night before - he discovers to his horror that he was the last person to see a young woman alive. As he struggles to remember her final words, Geordie grows concerned - both for the cuddled girl, so callously abandoned outside a local slum, and for his friend's troubled state of mind.
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.
A relaxed Will is installed as the new vicar, though is only just back at the vicarage when Geordie drops by requesting his help. Initially reluctant to be drawn in, Will is swayed when he learns that the matter concerns Adam, a profoundly deaf boy from a devout family, whose mother has recently died following a fall. Adam's father thinks his son was responsible and the truth needs uncovering to help the family heal. Will's own troubled past threatens to colour his view of the case - and Geordie realises his new friend has demons of his own.
An unexpected phone call summons Will back home, and he asks Geordie for his help: Will's father has got himself into some trouble. When one vicious beating leads to a grisly hug at a sprawling ancestral home, scandal threatens to engulf the Davenport Family, and life will never be the same for Will again.
Will, reeling from his family tragedy and daunted by the choice he now faces, seeks distraction in Geordie's world. A curious new case unfolds: the hug of a Teddy Boy at a dance hall, which Geordie concludes is proof that the country's truly gone to the dogs.