November 1966. Oxford's dreaming spires darken as Bonfire Night beckons. A housewife, Mrs Vivienne Haldane, found choked to death in her own home marks the third strangling in a month, setting Oxford City Police on edge. Pulled tight around the victim's throat -­--­- a silk black stocking. Though the victims bear little relation to one another beyond the superficial, DC Endeavour Morse bridges the divide: the three victims are married women, all of whom have been found shorn of their wedding rings. As a city-­-wide manhunt for a multiple murderer brews, Endeavour unearths a private diary kept hidden by Mrs Haldane, detailing periodic infidelities with an anonymous 'Mr X'. Her husband, a bloated mathematician, offers precious little information. But the specificity of the murderous stocking -­--­- a high-­-line brand, 'Le Minou Noir' -­--­- opens a new avenue of investigation. Endeavour traces the garment's provenance to Burridges, an innocuous department store, save for being the stocking's sole supplier across Oxford. As its denizens fall under suspicion, the investigation draws out painful memories for DI Fred Thursday, for whom a chance encounter with shopgirl Luisa Armstrong rouses reminiscences of wartime strife and unbidden promises.
As the suspect list narrows, and the Oxford Strangler stalks his next victim with increasing desperation, other common strands begin to pull into focus. But Endeavour and Thursday, grappling with personal travails in their private lives, miss a deeper tragedy unfolding, not far beneath the surface.