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.