Now Jaime sniffed. Theo sneezed. Tess rubbed her eyes, willing them to adjust. They were perched on some sort of landing. In front of them appeared to be nothing but the entrance to a stone tunnel leading . . . well, who knew where? Nine tugged at her harness, trying to pull Tess forward, though Tess held her back. This would have been easier on Tess’s arm if Nine didn’t weigh forty-five pounds.

Nine was definitely an Uppercase Cat.

“Well,” said Theo. “I don’t see the greatest treasure known to man, do you?”

“Nine can see,” said Tess. “And she wants us to keep moving.”

“Well, as long as the cat thinks it’s safe to keep moving,” said Theo.

“You mean the Cat,” said Tess.

“That’s what I said.”

“No, you didn’t.”

“I think the Cipher wants us to keep moving, too,” said Jaime. “Look down.”

Tess looked. At their feet, painted onto the stone floor, a phosphorescent-green arrow pointed straight into the tunnel. As if it had already anticipated their hesitation.

“Not particularly mysterious,” said Theo. “You’d think the Cipher would make things harder.”

“Haven’t things been hard enough?” said Tess. They had. Just over a month ago, a developer named Darnell Slant had finally persuaded the city to sell him all five Morningstarr buildings, the kids’ building among them. And just over a month ago, Tess and Theo and Jaime had decided that the only way they could save their building was to solve the greatest mystery of the Morningstarrs: the Morningstarr Cipher, the puzzle that the enigmatic twins had embedded in the streets and the monuments and the artifacts of New York City more than a hundred and fifty years before. A puzzle that people had been trying—and failing—to solve for all that time, because they had missed the most important aspect.

It hadn’t wanted to be solved. Until now.

At least, they hoped so.

“For all we know, the Cipher is leading us right into some kind of trap,” Jaime said. “It’s not like it would be the first time. Remember the Underway puzzle? When we almost died?

Theo said, “That’s comforting.”

“Always here to help,” said Jaime. Tess could tell that Jaime was trying to keep his tone bright, but he only sounded stressed. But then they were all stressed. And confused. They’d thought they solved the Cipher when the clues led back to their building. They’d thought they’d understood the Morningstarrs better than anyone. But the memories of their beloved building crashing down right before their eyes haunted them. It showed them that no one, not even they, really understood the Morningstarrs, and that the Cipher remained a cipher.

“Remind me why we’re doing this again?” Theo said.