By the time Henry, Anna, and José got up the stairs, Hem was standing in the middle of a swirling, paper-scrap mess. The window was wide open again, and the wind had freed a whole bunch of papers from their thumbtacks and Band-Aids.

Hem stood in front of the wall. “Where is it?”

“It’s … it was right here.” Henry pointed. “But it’s gone.” He kicked at the notes scattered on the wooden floor, but they were all written on scraps of paper. There wasn’t an orange napkin to be found.

“Was Ursa up here?” Hem’s eyes narrowed.

“She brought up pillows and stuff, but I doubt she would have taken the note.” Anna shook her head. “She saw it before and didn’t act like it was any big deal. I don’t think she even knew it was a message.”

“Who says it was?” Henry scoffed. “It didn’t even make sense.”

“It was. It must have been.” Hem paced back and forth.

Anna walked over to the window, pushed it shut, and turned the latches to lock it. “Did you lock this when you closed it before?” she asked José.

“I don’t think so. I’m not sure.”

Hem let out a frustrated sigh. “Someone must have come in while we were downstairs.”

“Who would want to steal a poem off the wall?” José asked.

“Someone who wants to intercept a Silver Jaguar Society message.” Hem looked out the window. “Someone with access to one of these apartments with windows on the courtyard.”

“Is that the only way onto this little roof?” Anna looked out the window.

Hem nodded. “Gilbert lives over there.” He pointed across to an apartment with spider plants hanging in the window. “We go through his apartment sometimes if the storefront is being watched and we don’t want to be seen.”

“Maybe he stole the note,” José said. “They think someone’s slipping information to the Serpentine Princes, right? What if —”

“No.” Hem shook his head. “Gilbert has been with the store — and the society — more then twenty years. There’s no way he’d do that.”

“That’s what I thought about my captain of the guards in Storm the Castle,” Henry said. “And then he totally turned on me. You never know.”

“Well, I know.” Hem frowned. “Someone else must have come across from a different apartment to nab that message.”

“But Henry’s right. How would somebody even know it was a message?” Anna asked.

Hem plopped down on the bench. “You know who Ixchel is, right? The Mayan goddess of creativity?”

“Of course,” Anna said. “The jaguar goddess.”

José nodded. “And a symbol of the Silver Jaguar Society.”

Henry flung his arm at the wall. “But there’s zillions of notes up there. You’re telling me somebody came in the window and happened to read that one and notice —”

“Yes.” Hem ran his hand over the sea of greetings and sketches, notes and quotes. “Somebody who knew what he was looking for, too. A napkin … and a false attribution.”

“Attri-what?” Henry asked. Hem threw around too many show-offy words.

“Attribution,” Hem answered. “A quote or poem with the wrong name attached to it is always a society message. John Milton never wrote anything like that poem you saw,” Hem said, rolling his eyes. “Any society member worth his salt would know that.”

Worth his salt? Henry had no clue what that meant, but he was pretty sure Hem was being a jerk. “Are you saying we’re not worth our salt? You know, we helped with two society missions before we even met you. We totally found the Star-Spangled Banner and the Jaguar Cup.” He stalked across the room to look for his GamePrism.

“Well then, you’re certainly the fellow for this job, aren’t you?” Hem smirked.

“Maybe we can reconstruct the message.” Anna jumped in, pulling out her notebook and a pen. “It started with, ‘Ixchel’s sons and daughters know … what’s found below is …’ ”

“ ‘What’s sought above is found below,’ ” José corrected, then went on. “ ‘Cizin’s nature there beguiles, those who step and dare to smile, where time’s a thief and dark eats light, where home is but eternal night … and …’ I didn’t read the end. Sorry.”

“Dude, how’d you remember all that?” Henry stared at him.

José shrugged. “I memorize a lot of quotes and stuff. You get good at it after a while.”

Anna was scribbling like crazy. “Cizin’s nature …” she mumbled, then looked up at the others. “Who’s Cizin?”

“Mayan god of death. He rules the underworld.” Hem looked at Anna and Henry. “Anybody remember how it ends?”

Anna shook her head.

Henry thought hard. “Something with X marks the spot.” That line had reminded him of his Treasure Quest game with Mad Ben the Pirate.

“ ‘Where all the past is laid to rest, the spot marked with imperfect X.’ That’s it!” Anna wrote it down and handed the notebook to Hem. “It’s a riddle, right?”

“They always are.” Hem frowned at the page, whispering the words to himself. For a while, they all watched him.

Finally, José yawned. “I guess we can show it to our parents when they get back.” He looked at his watch.

“What time is it anyway?” Henry asked. When were the adults going to show up?

“A little after eleven,” José answered. His voice was shaky, and Henry understood why. Were they supposed to go to bed in this weird bookstore by themselves? Why hadn’t Aunt Lucinda called? And why hadn’t his dad checked in? Was he still too busy at the hospital? That was hard to imagine … unless the baby wasn’t doing well. As soon as Henry considered the possibility, his annoyed feeling turned to shame. What if something really was wrong?

“Oh!” Hem said suddenly, bringing Henry’s thoughts back from the hospital. He held the notebook closer to his face, whispering the message again. “… who dare to smile …” He looked up at the others with his eyes wide. “I think this means they’ve got her!”

“Huh? Who’s got who?” Henry asked.

“Mum … and the others.” Hem tapped the page. “They stole the Mona Lisa. Not the Serpentine Princes.”

“What?” Anna’s mouth fell open. “They would never do that. They —”

“They certainly would if they felt it was the only way to keep her safe,” Hem said.

“Dude …” Henry’s jaw dropped. “You think your mom … and my aunt … and Anna’s mom and the McGilligans actually stole the Mona Lisa?” If that was true, it was cooler than any video game Henry had ever played. But it was impossible. “No way could they get away with that.”

“But there is,” Hem whispered. “Mum said they have society members working in restoration at the Louvre. They could have pulled it off. If they felt there was a threat —”

“If there are society members working at the Louvre, why couldn’t they protect her instead of stealing her?” Anna still looked horrified.

“Because they’re not the only ones inside the Louvre,” Hem said. “The Serpentine Princes have infiltrated half the museums in this city.”

Henry nodded slowly. “That makes sense. In Storm the Castle —”

“This isn’t a game, Henry!” Anna glared at him. “This whole idea is … stealing something to keep somebody else from stealing it first? I …” She stopped talking and let her words hang in the air. “Ohmygosh! This is just like …” She looked at Henry, and he knew exactly what she was thinking.

“Dude … it’s the Jaguar Cup all over again,” Henry said. A Silver Jaguar Society member in Costa Rica had stolen one of the society’s most sacred artifacts, just as it was about to be loaned out for an international museum tour, because he thought it would only be safe if it could be kept in Latin America.

“And we know how that worked out,” José said. “The cup was almost lost forever.”

Anna shook her head. “We can’t let them get the Mona Lisa!”

“I know.” Hem’s voice was calm, but his eyes looked wild. He pointed to the writing in Anna’s notebook. “We need to figure this out, and we need to get there before —”

“Get where?” Henry interrupted. He felt like they’d jumped from level two to level sixteen in a game and missed all the directions.

“Get … wherever this is sending us.” Hem held up the notebook. “I’m guessing …” He took a deep breath. “I’m guessing Mum and the others had to get rid of the painting quickly by stashing it somewhere … but now it’s not safe.”

Anna stared at the notebook. “You think this is trying to tell us how to find it? To save it?”

“Dude, you’re not serious.” If something like this showed up in one of Henry’s video games, he and his friends would laugh it right off the screen. Oh, look! A secret message … how convenient!

“You don’t get how serious this is.” Hem’s words came at Henry like bullets. “Maybe you haven’t noticed, but our parents haven’t come back. If they’ve taken the Mona Lisa … and then left us this” — he held up the notebook — “it’s because something has gone desperately wrong, and we’re the only ones who can get her to safety now.”

Henry didn’t know what to say. He looked at Anna and José. Anna’s face looked as white as her notebook paper, and José’s lower lip was trembling. Henry turned to Hem. “You really think that’s what happened?”

Hem nodded. He didn’t look full of himself anymore. He looked scared.

Henry took a shaky breath and tapped Anna’s notebook. “Then let’s try to figure this out.” He tried to sound brave, confident. Maybe he couldn’t do anything about the situation at the hospital back home. But he could do this. He could try — even though he had no idea where they were going, or who might be waiting when they got there.