The basement of the Louvre felt just like the crypt of Notre-Dame, all old stones and shadows.

“Is this a real dungeon?” Anna asked Hem, her pen poised over her notebook.

“Part of it. Come on …” He led them into a huge, circular room with crumbly walls. A modern wooden walkway snaked around the big center column of stones stacked upon stones. “The Louvre was originally built as a fortress in medieval times. This center tower was used as a prison and storehouse.”

Henry eyeballed the tower. It looked pretty darn solid, and he didn’t see any openings where somebody could hide a painting.

Hem’s eyes scanned the curving stone walls. “There must be some nooks and crannies here where you could —” He jumped back from where he’d been standing, looked around wildly, ducked under the railing, and wiggled himself into the crawl space beneath the walkway.

Henry peered over the railing. “Did you find it?”

Anna and José hurried over, too, but Hem shook his head and held a finger to his lips. “Pretend I’m not here. Hurry! Walk that way” — he tipped his chin in the direction of the entrance — “and listen.”

Henry looked toward the entrance. “Listen to what?” There was only a mom pushing a stroller through the entrance and, behind her, two guys with backpacks. “Who are those guys?”

“They’re nobody. Just do it!” Hem squirmed deeper into the shadows.

Henry raised his eyebrows at José, who shrugged and started walking toward the entrance.

But there wasn’t much to listen to. The mom was singing some quiet French baby song to her kid, and the two guys behind her were looking around. They looked like cartoon illustrations in a kids’ book about opposites. One was the size of a sumo wrestler, with a square face and a black mustache that looked like the letter M. The other was tall and skinny with blond hair, pale skin, and wiry glasses. He wore jeans and dusty work boots. Henry followed those boots with his eyes. Could they have made the footprints in the Notre-Dame crypt?

“Stop staring,” Anna whispered. She and José started talking about their favorite paintings from upstairs. Henry pretended to be fascinated with a map on the wall. But when he snuck another glance at the men, they were looking at that center column of stone. The sumo wrestler gestured and then pushed up his shirtsleeves, and Henry gasped. A tattoo of a snake — the symbol that Vincent Goosen’s thugs always had inked into their skin — slithered up the man’s wrist and disappeared under his sleeve. These guys had to be Serpentine Princes!

The men started walking again, and Henry turned quickly back to his map, trying to listen over the thumping of his heart. The men were arguing in French.

“Il n’y a rien ici!”

“Mais peut-être …”

The first guy made a noise like he was hacking up phlegm. “Non! J’avais raison. Vous êtes stupide. Allons au Panthéon!”

Henry heard footsteps moving away from him, and when he dared to look, he saw the skinny man run right into Anna.

“Excusez-moi,” the man grumped.

“Oh. Um …” Anna looked up at the ceiling for a few seconds. “Umm … de rien!” she said finally. But they were already gone. “Oh.” She looked at the empty doorway. “I can never remember French words fast enough.”

“What did they say?” Hem asked as he slithered out from the crawl space. He climbed back onto the walkway, brushing dusty rock crumbles off his pants.

“Dude, that big guy had a snake tattoo!” Henry’s heart was still running wild in his chest. “They’re Serpentine Prince members!”

“Obviously,” Hem said.

“You know them?”

“I’ve seen their photos on Mum’s computer.”

Henry felt like he might explode. “You said they were nobody! You hid and you left us out here with them when you knew they were Serpentine Princes? What if they recognized us?”

Hem laughed. “Three junior society members who look like they’re on a school field trip aren’t exactly what the Serpentine Princes are trained to worry about.” Hem waved his hand as if it were no big deal.

“Then why did you hide?” Henry glared at him.

“I’m a bit more high profile,” Hem said, “what with my mum’s position in the society here in Paris. I’m quite sure they would have recognized me.”

Leave it to Hem to think he was some kind of society rock star.

“Anyway,” Hem went on, “what did you hear?”

“They were arguing,” José said.

“Saying what?” Hem looked excited.

“The first guy said, ‘Blah blah blah,’ ” Henry said, still bristling. “And then the other guy said, ‘Blah blah blah I have a raisin and you’re stupide.’ ”

Hem blinked at him. “He said ‘I have a raisin’?” He turned to Anna. “Don’t you speak French?”

“Yes, but they were talking so fast! I only caught the very end. He said let’s go … somewhere. It sounded like panty hose, but that can’t be right.”

Hem squinted at her. “Allons au … panty hose?”

Anna bit her lip. “Maybe it was panty-hone?”

“Panty-hone …” Hem said, and then his face lit up. “Panthéon! Of course … the Panthéon! Come on!”

Henry, Anna, and José ran to keep up with Hem as they hurried out of the Louvre, over the bridge, and through the city streets.

“So what is this Panthéon place?” Anna asked, trying to catch her breath, when traffic finally slowed them down.

“Used to be a church.” Hem stopped at a corner. “Now it’s a mausoleum, where our country’s most distinguished citizens are entombed.” As soon as there was a gap in traffic, he started running across the street. Henry tried to follow and almost got taken out by a moped.

When he caught up, he said, “Another crypt? Seriously?”

“Yes, there’s a crypt. And a pendulum, and the message … time’s a thief … I can’t believe I didn’t think of it sooner.”

Anna grabbed Hem’s arm. “Wait, what?”

“Time’s a thief,” Hem repeated as he led them past a little café. Henry looked longingly at a tray of ham-and-cheese baguettes behind the glass window. It had to be lunchtime, didn’t it? Were they not going to eat until they found the stupid Mona Lisa?

“That time reference could very well be talking about the pendulum at the Panthéon.” Hem ducked in front of a taxicab and jogged across another intersection, toward a huge building with tall stone pillars all across the front.

“The what at the who?” Henry was tired of all the dumb riddles. He was tired of this stupid, follow-Hem-because-he-knows-everything race through the city. He wanted a sandwich and a crepe and a nap.

“Just come on.” Hem raced up the stairs and started toward the doorway. “None of this will matter if we don’t get there first.”

“No.” Henry stepped in front of him. “You lied to us back at the Louvre. You’ve got us running all over the city. I can’t understand a word anybody’s saying. I almost got hit by a motorcycle, and —”

“It was only a moped,” Hem interrupted.

“I don’t care what it was!” The tourists hanging around on the steps were starting to stare, but Henry didn’t care. “I’m not going anywhere else until I know what’s going on.”

“Then don’t come. We can do without you quite nicely.” Hem started to push past Henry.

This time, Anna stepped in front of him. “No. We’re in this together until our parents come back. All of us. But Henry’s right. You need to tell us what you’re thinking so we can help.”

“Fine.” Hem sighed and plopped down on a step. Henry, Anna, and José lined up beside him — they looked like the pigeons perched on the edge of the bakery roof across the street — and leaned in to listen.

“The part of the message that said ‘Where time’s a thief’ made me think of this place.” Hem gestured toward the doors above them. “Foucault’s pendulum is here and so is the Wagner clock,” Hem said.

“Who are Foucault and Wagner?” Henry jumped in. “Those guys from the Louvre?”

“No.” Hem rolled his eyes. “Foucault was a French physicist who created a pendulum clock model to prove that the Earth rotates. And Wagner was a clockmaker who came up with a particular kind of clock mechanism with gears and whatnot. Now can I finish?” Hem’s voice had an edge. “You have to understand that the UX has a long history with the Panthéon. That’s why this is so perfect.”

“Wait …” Anna was having trouble keeping up in her notebook. “That’s that Urban eXperiment group?”

“Right,” Hem said. “The UX broke into the Panthéon a few years back, set up a secret workshop, and spent a year restoring the old Wagner clock.”

“They break into buildings to fix stuff?” Henry couldn’t help interrupting. That sounded kind of cool.

Hem nodded. “They’re dedicated to restoring parts of Paris history that the government doesn’t seem to care about anymore.”

“They sound like the Silver Jaguar Society,” Anna whispered.

“Indeed. There’s a lot of overlap between the groups. So that’s why I thought … time’s a thief …” Hem looked at the doors. “I wanted to get here before those blokes from the Louvre.”

It kind of made sense. If it was all true. “All right.” Henry stood up. “Let’s go.”

Hem led them past the volunteer at the ticket desk into the lobby. The ceiling was high, vaulted, and billowy. The walls had huge murals and elaborate sculptures carved right into the stone, and in the middle of the floor was a big ring with numbers and lines, like a giant tape measure wrapped in a circle. Above it, a shiny golden ball swung back and forth on a long, straight wire. Henry tipped his head back to see where it was attached, at the very center of the domed ceiling overhead. A circle of tourists stood around it, watching the ball swing. It was kind of mesmerizing, like some giant hypnotist was holding the wire way up there.

They all watched the shiny ball swing back and forth until José broke the spell. “There’s nowhere to hide anything here.”

Anna looked around. “Where’s the clock?”

Hem showed them a simple-looking old clock, way up high. There was no place to conceal a painting there either. “Let’s go downstairs.” He leaned in toward Henry, Anna, and José and whispered, “That’s the only place you could stash anything, really.”

They hurried downstairs and through a chilly room with a bunch of fancy, roped-off casket things. Each one had a statue of a guy in front of it.

“Oh!” José hurried over to one. “Voltaire is here? I love Voltaire!” He stared up at the statue, which was holding a notebook in one hand and some feathery pen in the other.

Henry turned to Anna. “He looks like the guy version of you, a few hundred years ago.”

José made a face, as if Henry had offended the statue. “Don’t you know who Voltaire is?” José struck a philosopher pose with his index finger sticking up in the air. “He said, ‘Common sense is not so common,’ and ‘Every man is guilty of all the good he did not do.’ ”

“Whatever,” Henry said, looking around. “Where’d Hem go?”

“Maybe over here?” Anna led them down a narrower hallway to the left.

“He probably ditched us. He said he’d rather be on his own,” Henry said, following her.

“Well, that’s not an option, because we’re helping.” Anna stopped at a sign. “This must be a list of who’s entombed here.”

José read the names as if they were old friends. “Victor Hugo … Alexandre Dumas … Émile Zola …” It reminded Henry of the list of residents posted by the doorbells at their apartment building, only these tenants were all dead.

“Look who else is here.” Henry pointed through the crowd, where Hem was wedged into one of the arched doorways. He must have climbed behind the iron railing that separated the tombs from the tourists, and he was tucked against the stone wall, peeking out.

“Good,” Anna said, and started down the hallway.

But Henry grabbed her arm. “Wait a second …”

Henry, Anna, and José stayed back, watching as Hem leaned out his doorway a little. He tipped his head toward a cluster of people down the hall, his eyes cast up at the ceiling, listening.

“It’s them!” Anna hissed, pointing. “Those Serpentine Prince guys from the Louvre!”

José stepped back as if he wanted to melt right into the stone wall, but Henry leaned forward to see.

The museum guys were standing in a cluster of people looking at whatever was in the next little room, but their attention wasn’t on the tomb. They seemed to be arguing again.

“Doesn’t look like they found anything here either.” They were right back where they had started, and Henry was starting to feel like this stupid painting search would never end. It was like the worst video game ever, and they couldn’t even get past level one. They needed a cheat code or something. Actually, what Henry really needed was lunch. He turned to Anna and José. “Let’s go back to the bookstore.”

But Anna held up her finger and looked past Henry, down the hall. Henry turned and saw the two Serpentine Prince guys walking away. Hem peeked out from his hiding spot, flung one long leg over the railing and then the other, and followed the guys toward the exit.

“Come on.” Henry pointed back toward the big room with José’s statue buddies. “Let’s go this way and meet Hem outside. We can see what he has to say about all this.” Henry couldn’t help adding, “But I doubt we’re going to hear the truth.”

With all Hem’s hiding and lying and eavesdropping, Henry had a bad feeling about him. A worse-than-usual feeling. And that was saying something.