“Maybe Dr. Couture stole them from Bull,” Lacoste suggested.

“Okay, let’s say he stole them, then what?” said Beauvoir. “Couture just hides them in his home. Why not sell them if they were that valuable?”

“Maybe he wanted to make sure no other gun was ever built,” said Cohen.

“Then why not destroy the plans?” asked Beauvoir. “Why keep them?”

“We don’t know that he did keep them,” Lacoste pointed out. “We’re pretty sure he didn’t sell them because no other gun was ever built, but he might’ve destroyed them. We don’t know, and the killer wouldn’t know either.”

“But that would mean the murderer knew about the connection between Antoinette’s uncle and Gerald Bull,” said Reine-Marie. “Why didn’t he look for the plans sooner? Why now?”

“Because the gun was found now,” said Lacoste. “That’s the catalyst. Until then the plans were worthless. But once a working model was found—”

“The plans become priceless,” said Reine-Marie. “I get it.”

“There is another possibility,” said Gamache. “That Gerald Bull never had the plans.”

They stared at him. They’d moved from the soup to fettuccine with grilled salmon, tossed with fennel and apple.

“His apartment in Brussels was searched several times before his death, and nothing was found,” Gamache explained. “After his murder, people looked but the designs for Project Babylon never turned up. It was assumed that Bull, knowing he was in trouble, destroyed them. But suppose they weren’t found on him because he didn’t have them?”

“Because he gave them to Dr. Couture,” said Lacoste.

“Or because Couture stole them,” said Beauvoir.

“Or,” said Gamache, “because Gerald Bull never had the plans to begin with.”

“You think Gerald Bull was the ‘front of house,’” said Lacoste. “And Dr. Couture the real genius?”

“I think it’s possible Dr. Bull did not have the plans because Dr. Bull did not make the plans,” said Gamache. “Do you still have the photograph, Adam?”

Agent Cohen jumped up and returned a moment later with the picture. He put it on the kitchen table and everyone leaned in.

“Do you want more light?” asked Reine-Marie.

“No, this is fine,” said her husband. The candlelight was indeed soothing. “I think they might have been a perfect team,” he said, looking at the photo. “Bull gregarious, outgoing. Dr. Couture quieter, a bachelor scientist. Devoted to his work.”

“His work being Project Babylon,” said Beauvoir.

“According to what you found out”—Gamache turned to Cohen—“Dr. Couture started working with Gerald Bull at McGill, on HARP.”

“Right, but funding for the High Altitude Research Project was cut,” said Cohen. “And Dr. Bull left McGill.”

“What did he do then?” Gamache asked.

“He formed the Space Research Corporation,” said Cohen.

“And the SRC eventually developed the long-range artillery that became Project Babylon,” said Lacoste. “By then it was a private company, run by Bull.”