The master of Wisbech Grammar School has been found dead by poisoning. In the eyes of the local sergeant, only one person can be considered guilty; schoolboy Rupert of Tilneye. With the youth secured in the cells of Wisbech Castle, it looks certain that he will hang for the crime. The seneschal for the Isle of Ely, asked to investigate by the Tilneye family, soon starts to delve into the matter. The number of suspects, hidden motives and opportunities that he uncovers, however, refuses to be narrowed down. He realises that the only way to solve this mystery is by setting a trap. This is 1423, a time of the religious guilds, when Wisbech is run by the most prominent fraternity of them all, the Guild of the Holy Trinity. These are the early days too of the town's grammar school, situated in a single room above the porch of St Peter's Church. The seneschal must negotiate layers of local authority, prejudice and hidden agendas. He must even bend a few rules before he can discover the truth.