Retired archaeologist Judith Potts (Bond) 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 neighbor’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 (Martin) and unfulfilled vicar’s wife Becks (Horgan) as they start an investigation of their own.