An investigator comes out of retirement.

A woman is found dead in a hotel room on the east side of Manhattan. Her face and teeth are gone, the room has been sprayed with antiseptic, and there is acid in the bath. The method comes from a book on forensic investigation published under a false name that sold very few copies.

The man who wrote that book is called Pilgrim in the narration. He ran a secret American intelligence division that has since been closed down. He lives in New York under another name and does not work.

A detective, Ben Bradley, finds him and asks him to look at the hotel room. Pilgrim looks at it and tells Bradley how it was done. Bradley carried Pilgrim out of the North Tower on the day it fell, which is why Pilgrim answers the door.

The book then goes backwards. Pilgrim is adopted by a wealthy family in Greenwich after his mother dies. He is recruited out of college. He runs an operation in Moscow and another in Bodrum. He kills a man in a Geneva hotel with a needle and takes a set of documents out of the room.

A second thread starts in Saudi Arabia. A boy of fourteen watches his father beheaded in a public square in Jeddah for speaking against the royal family. The boy grows up, trains as a doctor, fights in Afghanistan against the Russians, and then works in a hospital in Lebanon. He is called the Saracen. He decides that the way to bring down the House of Saud is to attack the country that supports it.

He tests a modified smallpox virus on three men in a hut in Afghanistan. All three die. He needs a laboratory, a stock of vaccine to work backwards from, and a route into America.

In Washington the President is told. Pilgrim is brought back in. There is one lead: a phone call, a mirror in a hotel, and a name connected to a death in Turkey. Pilgrim is sent to Bodrum with a badge that says he is an FBI agent investigating that death.

In Bodrum he meets Cumali, a local police officer with a young son, and a group of wealthy Americans staying at a house above the cliffs. A man called Dodge went off those cliffs during a party. Pilgrim looks into that death while looking for something else. He searches a house at night, finds a hidden room, and is arrested and then released.

The two threads meet through a birth certificate and a mosque.

The final section runs across several countries in a short space of time. There is a sequence in a Turkish prison, a sequence on a mountain road, and a sequence at a hotel in the desert. The Saracen reaches New York and rents laboratory space. A school group is involved by accident.

Pilgrim works out where the vaccine is going and how long there is. He calls it in from a payphone.

The last chapter is Pilgrim on a beach in a country he does not name, writing all of this down.