They left the bookstore and hurried down the bustling sidewalk and were halfway to Notre-Dame before it occurred to Henry that he didn’t totally understand just what they were looking for. He edged closer to Hem. “Do you think this painting will still be in its frame? I mean, what should we be watching for when we get to the crypt?”

Hem led them across the street and hopped up onto the curb. “First, I don’t think we’re going to find anything here.” He dodged a lady pushing a stroller. “Second, I have no idea how the piece was taken, if it’ll be flat or packaged up in a box or what.”

“Well …” Anna was jogging to keep up with Hem’s long stride. “We’ll just have to look for anything suspicious.”

“Or not,” Hem stopped abruptly at a stone wall and leaned over to point at a sign below.

CRYPTE ARCHEOLOGIQUE.

FERMÉ.

“Oh! That means closed,” Anna said.

“Doesn’t open for another twenty minutes,” Hem added.

“Well, that’s lame,” Henry said. Seemed like any secret society member worth his salt would be able to find a way in to investigate. If this were a video game, they’d bust down a back door or rappel over a wall. But Henry looked around and couldn’t find any secret entrances or ropes. He didn’t even have his GamePrism. “Anything to do around here while we wait?”

“I’m going to take a nap.” Hem shooed some pigeons off a bench and stretched out. “You can go see the cathedral if you want. That’s open now.” He waved his hand in the direction of the big church across the square, then put his fedora over his face.

“Oh, let’s do that! I’ll write a travel feature about it for my school paper.” Anna pulled her notebook from her backpack.

“One thing.” Hem lifted the hat from his face. “Watch for pickpockets.” He wiggled his fingers. “They love you tourists.”

Henry rolled his eyes and turned to José, who had been quiet all morning. “You coming?”

“Sure,” José said as they started walking. As they got closer to the huge cathedral, he turned to Henry, blinking fast. “Do you think they’re okay?”

“Yeah.” Henry had actually been trying not to think about Aunt Lucinda and the other grown-ups. “They’ve been on tons of other missions.”

“And they’re not alone,” Anna added. “Mom says there are lots of society members in Paris, so even if something did go wrong, they’ll … they’ll get help.”

José nodded, and they tagged onto one of the lines snaking toward the cathedral doors.

From across the square, Notre-Dame Cathedral had looked like a big letter H with the bottom space filled in. Henry thought it was cooler up close. If you looked straight up, these drooly stone gargoyle things stared down at you from the roof.

“Are you going in?” Anna nudged him from behind, and Henry realized the line had moved. He hurried through the cathedral’s tall doorway. And stopped.

Anna did, too. “Ohh … it’s beautiful,” she whispered.

“Yeah.” Henry stood still and took it all in, from the giant stone columns and arches to the brilliant stained-glass windows above the cross at the front of the church. It felt like staring into the Middle Ages.

“How old is this place?” he whispered, turning back to Anna and José. But they’d already wandered off. Anna was halfway up one of the aisles, scribbling in her notebook. On the other side of the church, José stood in front of a tall statue and a big, round wiry thing that held a bunch of little candles, all lit up.

Henry walked up to him. “Hey.” He looked down at the unlit candle in José’s hands. “What are you doing?”

“I was going to light a candle for my mom and dad,” José said, “and your aunt and everybody, too. My grandma does it all the time at her church in Vermont.” He paused. “It’s supposed to … I don’t know … help prayers work better, I guess. But I don’t have two euros.” He glanced up at a sign asking for the donation.

“Well,” Henry said. He didn’t know much about stuff like this. “We need help, so I think we gotta figure God won’t mind.” He peeked in the donation bowl, which was full of coins. “Besides, God’s already doing pretty well this morning.”

José nodded and moved to the next statue. “I wish there were a patron saint of kids whose parents might be missing.”

“You guys!” Hem hissed from near the entrance. “The crypt’s open. Let’s go!”

“Just choose one,” Henry said.

José lit a candle next to the Joan of Arc statue, then looked up at her. “We’d appreciate your help. Please look after our parents and Henry’s aunt and bring them back safely.”

José started toward the exit, but Henry paused and looked back at the statue. “If you could help out my dad and Bethany and the baby, too, that would be great,” he whispered, and followed José out into the sunlight.

“What took you so long? Come on!” Hem started across the square. They dodged tourists, scared away a flock of pigeons, and hurried down the stairs to the entrance of the crypt. Hem led them past the tiny gift stand and some diorama displays, and along a railing that overlooked a pit full of old bricks and columns. They took a few steps down, and the air changed. It was cool and musty-damp, like rotten leaves.

Hem stopped to peer over another railing into the excavation pit but shook his head at the dirt and stones. “I’m not seeing anything.”

“What is all this stuff?” Henry asked. It was nothing like the crypt of the Old North Church, with its carefully labeled tombs.

“History,” Hem said impatiently, hurrying past some old oven-like thing. It reminded Henry of the place his dad used to take him for wood-fired pizza in Vermont before Bethany came along and they had to move. Hem rushed them past some more ruins. “The arches and stacks of rocks are from ancient Roman baths, but I don’t think —”

“Hey, look!” Henry pointed through the bars of the railing, into the loose dirt next to one of the oven-things. There were footprints. And not from some crazy old fossilized ancient Roman. Modern footprints. “Those look like work boots.”

Hem squatted down, peered into the shadows, then shrugged. “It could be a worker who was changing a lightbulb or something.”

“Maybe,” Henry said. There were buttons on the sign to make different parts of the ruins light up. Henry pressed one. “But I don’t think so, because look …” He pointed, moving his hand all over the pit as the lights changed. “Those prints aren’t only where the lights are. They go all over the place, like somebody was looking for something.”

“I doubt it.” But Hem kept staring, frowning. Then he glanced up and down the empty walkway, climbed between the metal bars of the railing, and hopped into the archaeological site.

No alarms went off or anything, but Anna gasped. “Get back up here!”

José looked around. “No one seems overly concerned about his trespassing.”

“What’s he gonna do? Steal a rock?” Henry watched Hem bend down to inspect the footprints. He got on his hands and knees and started to crawl into the oven-thing. But he didn’t get far.

“Anything in there?” Henry called down.

“No.” Hem coughed, backing out of the archway. He stood up, returned to the walkway, and climbed back up, brushing old dirt off his knees and hands. “I don’t think there was ever anything here.” But he squatted again, squinted into the arch of the oven-thing, and frowned.

Henry did the same thing and understood. From the walkway, it looked like the arch might be a tunnel, like it might go way deeper than it did. Like maybe it was the kind of place you could hide something. “You think somebody else was looking for the Mona Lisa here?”

“Shhh!” Hem said. Then he nodded. “Yeah.”

“Ohmygosh!” Anna’s eyes were huge. “Whoever came in the window and took the napkin must be —”

“Shhh!” Hem headed for the neon exit sign, and they followed him up the stairs into the open square. “Well, we know one thing now.” Hem folded his arms in front of him, as if he were cold, even though the sun was beating down. “We’re not the only ones trying to solve this riddle.”

José frowned. “But the painting wasn’t there, so —”

“We got lucky. Whoever came here before us guessed wrong, too.” Hem took a deep breath. “Next time, we need to get it right. Before someone else does.”

“This is getting creepy,” Anna said.

Henry nodded. It was like some weird multiplayer video game where you didn’t even know who your opponent was. “So now what?”

Hem raised his eyebrows. “How do you feel about dungeons?”