It is the early spring of 2016 and Queen Elizabeth is at Windsor Castle in advance of her 90th birthday celebrations. But the preparations are interrupted when a guest is found dead in one of the Castle bedrooms. The scene suggests the young Russian pianist strangled himself, but a badly tied knot leads MI5 to suspect foul play was involved. The Queen leaves the investigation to the professionals—until their suspicions point them in the wrong direction. Unhappy at the mishandling of the case and concerned for her staff's morale, the monarch decides to take matters into her own hands.

In a peaceful retirement village, four unlikely friends meet weekly in the Jigsaw Room to discuss unsolved crimes; together, they call themselves the Thursday Murder Club. When a local developer is found dead with a mysterious photograph left next to the body, the Thursday Murder Club suddenly find themselves in the middle of their first live case.

Judith Potts is 77 years old and blissfully happy. She lives on her own in a faded mansion just outside Marlow, there’s no man in her life to tell her what to do or how much whisky to drink, and to keep herself busy she sets crosswords for The Times newspaper. One evening, while out swimming in the Thames, Judith witnesses a brutal murder. The local police don’t believe her story, so she decides to investigate for herself and is soon joined in her quest by Suzie, a salt-of-the-earth dog-walker, and Becks, the prim and proper wife of the local vicar.

At the end of her first unsuccessful season out in society, Lady Georgiana has all but given up on attracting a suitable man - until she receives an invitation to a masked Halloween ball at Broxley Manor. Georgie is uncertain why she was invited, until she learns that the royal family intends to marry her off to a foreign prince, one reputed to be mad.

At age 26 Agatha Raisin has already come a long way. She has clawed her way up since leaving the Birmingham slum where she was born. She's lost her Birmingham accent, run away from her drunken husband, and found a job at a public relations office as a secretary. Then her boss asks Agatha to go to the home of Sir Bryce Teller to tell him that he is soon to be arrested for the murder of his wife and that the agency no longer wants to represent him.