But even with what Larkin had learned from his conversation with Reynold, he was still left with a multitude of unanswered questions: Who pushed Marco in front of an incoming Q train twenty-three years ago? Why had Archie suddenly ceased any and all communication—and before Larkin was ever put on his trail? If that John Doe in the IKEA tote was in fact Dicky, who killed him? What was their reason? Why had the photograph addressed to Larkin been left on his person?

It felt a little like a game of sudoku.

Larkin had, for over a month now, been watching Doyle work his way through a book of ever-increasingly difficult puzzles each morning. Doyle had an eye for detail, of course, but also the ability to comprehend the bigger picture. It was a particular kind discipline that made him so suited for forensic artwork. He understood how to capture unique elements when working on a composite sketch, but he managed his time and was mindful that his job was to construct an entire portrait and not to spend three hours capturing a certain glint a suspect had in their eyes.

Larkin had backseat-helped for a few puzzles. He’d been overly confident while pointing to an empty square and explaining, based on the established results of the vertical column, the only possible answer was four, and Doyle would smile kindly, sweetly, drag his pen along the horizontal column that Larkin had overlooked, and point out that four was already in play. After Larkin had made the same mistake a few times, he stopped trying to beat Doyle at sudoku.

It wasn’t that Larkin couldn’t step away and take in the scene as a whole. It was only that by his very nature, he hyperfixated on the smallest details first. So the questions he had regarding their ongoing investigation were like his preoccupation with trying to fit that four into a box it wasn’t meant to be in. Maybe by finding that one right clue, Doyle would be able to point out that the four was really a six and Larkin would be able to make all the remaining pieces fall into place.

“I think it would be irresponsible of us to write off anything as a potentially useful clue at this point,” Larkin concluded.