Retired archaeologist Judith Potts lives alone in a faded mansion in the peaceful town of Marlow, filling her time by setting crosswords for the local paper. During one of her regular wild swims in the Thames, Judith hears a gunshot coming from a neighbour's garden and believes a brutal murder has taken place.

When the police are reluctant to believe her story, Judith finds herself forming an unlikely friendship with local dog-walker and empty-nester Suzie and unfulfilled vicar's wife Becks as they start an investigation of their own. Eventually asked to assist with the official police investigation, headed by newly promoted Tanika, the women must piece together clues, grill suspect witnesses, and face down real danger as they work against the clock to stop the killer in their tracks.