A priest is murdered in his rectory. Patrick Casey, the protagonist, is assigned this case because he attended a seminary, and he is a Catholic. The rest of the office is Jewish and Wasps.
There is a court scene where a high ranking member of the clergy is a witness. Patrick Casey knows what Mental Reservation is, i.e, any member of the clergy can swear an oath but if he says silently, to himself, a little prayer, oath, he has sworn nothing and in essence can lie like a rug. He can say anything, even under judicial oath, that is what mental reservation is. There is a difference between a religious and a legal oath. If he pronounces a formula that expresses an oath, without the intention of swearing, then he has sworn nothing.
Patrick Casey knows about this formula and he confronts the Bishop with this. He explains this to the jury.
The seemingly drop-dead guilty defendant, Stanislov Kesky, is found not guilty. The priest, Bishop, knows this, he also knows that the Curia has its assassins.
The story goes back to WWII and Kesky's father, a Ritter Croix winner. It goes through the cold war and the Prague spring and ultimately to America.
Patrick Casey is assigned to defend Kesky; he is successful.
Patrick falls deeply in love. She becomes pregnant. She has a brain tumor from a mugging which Casey saved her from, he thought.
The church has the only cure available, but Susan must become a recluse in a nunnery in Quebec.
The assassin finds out that he has prostate cancer, and there's no hope. He has a change of heart and wants to do one right thing before he dies. He allies with Patrick. They raid the nunnery and there is a shoot-out, and they rescue Susan.
A radio report on the car radio has a breaking news big item: the Pope and one of his major henchmen are assassinated by using a rocket propelled grenade (RPG).