Late February saw him in Mexico City, where a bribe produced a lead that sent him to Panama. He spent a week there watching an empty condominium on the Playa Farallón. Then, on a hunch, he flew to Rio de Janeiro, where a plastic surgeon with a dubious clientele admitted he had recently altered the target’s appearance. According to the doctor, the patient claimed he was living in Bogotá, but a visit there turned up nothing but a distraught woman who might or might not have been carrying his child. The woman suggested he look in Buenos Aires, which he did. And it was there, on a cool afternoon in mid-April, that an old debt came due.

He was cooking at a restaurant called Brasserie Petanque, in the southern barrio of San Telmo. His apartment was around the corner, on the third floor of a building that looked as though it had been plucked from the boulevard Saint-Germain. Across the street was a café where Keller sat drinking coffee at a table on the pavement. He wore a brimmed hat and sunglasses; his hair had the healthy sheen of a man gone prematurely gray. He appeared to be reading a Spanish-language literary magazine. He was not.

He left a few pesos on the table, crossed the street, and entered the foyer of the apartment building. A tabby cat circled his feet while he read the name on the mailbox for Apartment 309. Upstairs, he found the door to the apartment locked. It was no matter; Keller had acquired a copy of the key from the building’s maintenance man for a bribe of five hundred dollars.

He drew his gun as he entered and closed the door. The apartment was small and sparsely furnished. Next to the bed was a pile of books and a shortwave radio. The books were thick, weighty, and learned. The radio was of a quality rarely seen any longer. Keller powered it on and raised the volume to a whisper. “My Funny Valentine” by Miles Davis. He smiled. He had come to the right place.

Keller switched off the radio and moved aside the curtain that shaded Quinn’s last remaining window on the world. And there he stood with the discipline of a close-observation specialist for the remainder of the afternoon. Finally, a man appeared at the café and sat at the same table Keller had vacated. He drank local beer and was dressed in local clothing. Even so, it was clear he was not a native of Argentina. Keller raised a miniature monocular telescope to his eye and studied the man’s face. The Brazilian had done a fine job, he thought. The man at the table was unrecognizable. The only thing that betrayed him was the way he handled his knife when the proprietor brought his steak. Quinn was a master technician, but he always did his best work with a knife.