When two nuns die in mysterious circumstances at St Agnes convent, Father Brown investigates with the aid of Sister Boniface, an eager young fan of detective novels.

The grand opening of Pryde Castle ends in disaster when a guide is struck by an arrow. Contrary to Inspector Sullivan, Father Brown becomes convinced that the motive lies buried in the family's past.

Father Brown finds himself immobilised with a broken leg when a member of Kembleford's WI dies in mysterious circumstances. Can he crack the case armed only with a telescope and the gang to help?