AtMidsomerPriory,hometoasmallgroupofnuns,MotherThomasAquinasisstrangledand,shortlyafterwards,asetofsilverworth￡60,000isfoundtobestolenfromthesafe.SisterCatherine,themostrecentmemberoftheorder,turnsouttobedescendedfromSirAnthonyVertue,whodonatedthepriorytothenunstwocenturiesearlierandBarnabylearnsthatiftheorderisendedshewillinheritthelandandbuildings.FatherBehan,whoheardthenuns’confessionsandhadlongexhortedMotherJulian,theprioress,tosellthepremises,isalsokilledandthemissingsilverturnsup,notstolenbutsoldbyMotherThomasAquinas.ThenBarnabylearnsofanatrocityinAfricathirtyyearsearlierwhereMotherJulianwasamissionary,whichcouldhelphimsolvethecasebutalsoputanendtothesacredtrust.