Seventy-seven-year-old Judith Potts is blissfully happy. She lives alone in a faded mansion in Marlow, sets crosswords for The Times, and there's no man in her life to tell her what to do or how much whisky to drink.

One evening, while out swimming in the Thames, Judith witnesses a brutal murder. When the local police don't believe her story, Judith and two unlikely friends decide to investigate for themselves. Together, they are the Marlow Murder Club.

But soon another body turns up, and it seems they have a real-life serial killer on their hands. Now the puzzle they set out to solve has become a trap from which they might never escape...