Cork, Ireland. 1923. When, one wet March morning, Reverend Mother Aquinas discovers a body at the gate of the convent chapel washed up after a flood 'like a mermaid in gleaming silver satin', she immediately sends for one of her former pupils, Police Sergeant Patrick Cashman, to investigate. Dead bodies are not unusual in the poverty-stricken slums of Cork city, but this one is dressed in evening finery; in her handbag is a dance programme for the exclusive Merchant's Ball held the previous evening - and a midnight ticket for the Liverpool ferry.
Against the backdrop of a country in the midst of Ireland's Civil War, the Reverend Mother, together with Sergeant Cashman and Dr Sher, an enlightened physician and friend, seek out the truth as to the identity of the victim - and her killer.