It is Holy Week at Westminster Abbey and preparations have begun for some of the most important few days in their year. Music Director James O'Donnell has to get the choir into perfect tune. Meanwhile, Matthew Payne, Keeper of the Muniments (the abbey's vast collection of documents), turns detective and investigates something mysterious.

Lindsay Hoyle, Speaker of the House of Commons, pays a visit to the abbey, and meets with his friend and colleague, Canon Tricia Hillas. She is chaplain to the speaker, and Tricia takes Lindsay on a tour of the Chapter House.