1929. Police surgeon Dr Clive Mason is devastated when his wife Martha is found dead in an empty horsebox at Southwell Races, her handbag stolen. The Masons were enjoying a day out at the races with their friends, Dr Ephraim Phillips and his wife Nora, when Martha went to speak to a face she recognized in a crowd - and disappeared.

As Detective Chief Inspector Henry Johnstone and Sergeant Mickey Hitchens investigate, it's clear this wasn't a robbery gone tragically wrong - Martha was deliberately murdered. Why did the life of this well-respected, popular member of the community meet such a tragic end? Henry and Mickey soon learn that the Masons' marriage was not what it seemed, and make a number of shocking discoveries. Who was Martha Mason, and what secrets was she hiding?