The brothers of the abbey of St Martin's-in-the-Marsh pay little heed to the tales of robber baron Sir Geoffrey Mandeville's ghost galloping through the Lincolnshire fens with a retinue of ghastly horseman. They may hear the shrill blast of a hunting horn, or see the corpse candles glowing in the dark, but their comfortable life is protected by a high wall and their powerful abbot. Until Abbot Stephen, a friend of the King, is found dead and Sir Hugh Corbett, Keeper of the King's Seal, arrives to investigate.