Blue Moon is a small retirement home where residents enjoy their twilight years. There is a harmonious atmosphere until a vacancy is filled by Rose Johns, who quickly reveals a malevolent character. Unrest and irritation among residents and staff soon arise, with even a suggestion of blackmail.

When a resident dies in suspicious circumstances, Detective Inspector Matt Devlin and his partner Detective Sergeant Rick Preston are called in to investigate. Their work is made considerably more difficult by having to allow for the foibles of several residents. To complicate matters further, Esme Harvey, Devlin's girlfriend, a freelance bookkeeper, spends two or three days a week at the home, so she knows both the residents and staff.

With a surfeit of suspects, Devlin and Preston face a formidable task. Can they catch the killer before they strike again? And will the interests of the residents help or hinder them in their mission?