Petra, Calder, and Tommy trudged the two blocks from the University School to Mrs. Sharpe’s house on Wednesday afternoon. It was a cool March day, gray with a smudge of green. Calder’s pentominoes clacked in his pocket and Petra was silent. The sidewalk wasn’t wide enough for three, so Tommy walked behind, realizing that as a group they looked close to pathetic — either stick-thin with nasty hair, dumpy, or boasting a pimple that belonged in the Guinness World Records book. Well, there was no way out now. Mrs. Sharpe and Ms. Hussey would be waiting, as well as these two super-accomplished kids.

Why hadn’t the three just said no?

Even as he thought it, Tommy knew none of them could ever say that to Ms. Hussey, who had been their better-than-best sixth grade teacher. She was brave, unpredictable, and often made decisions with her class — even if they didn’t always work out. She fessed up to mistakes and she never seemed bored, which matters when you’re a kid sitting for hours at a desk. Sometimes she wore clothes with weird colors or exciting patterns to school — Tommy remembered a skirt covered with exploding firecrackers and a pair of pants with diving mermaids on the back pockets. Sometimes she even brought them doughnut holes, breaking all the rules. Plus, she was a loyal friend to confusing old Mrs. Sharpe, who had gotten to know the kids and helped them through some of their most difficult and thrilling investigations.

The thing about Mrs. Sharpe, Tommy thought, was that you were never quite sure where the mystery stopped and she started. You knew she was hiding stuff, you could feel it, and you were never 100 percent sure it was for the right reasons.

You never knew who was fooling whom.

Mrs. Sharpe lived in a Victorian house that was gray with plum trim and boasted a huge wisteria vine over the front porch. Without leaves, the vine looked threatening, more like a boa constrictor than anything else. The house was bigger, fancier, and less homey than the places the kids lived. Plaster lions reared up from the garden as you walked to the front entrance, as if reminding you to feel uncomfortable.

The door flew open after one knock. It was dim inside, and Ms. Hussey’s voice was subdued. “She’s not feeling too much like herself,” she half whispered to the kids. “Come on in.”

“Uh-oh,” Calder said.

As they stepped in, Petra reached quickly into one pocket and pulled out a hair clip. Tommy didn’t see her elbow coming and it poked him right in the eyebrow, the one located beneath the volcano.

“Ow!” he yelped. Calder turned around just as Petra stepped to one side, and the two bumped bellies. Both moved away in the same direction, pressed together in a dreadful dance.

Petra was the first to recover. “Hello, Mrs. Sharpe,” she said. “We’re here,” she added … and then wished she hadn’t.

Mrs. Sharpe’s voice rose from a small red velvet sofa in the corner.

“That is evident. I should have known you three would arrive in style.”

The old bag sure knows how to make a bunch of kids feel at home, Tommy thought to himself.

Immediately Ms. Hussey cleared her throat with a businesslike chuh-chuh-chuh-hum and introduced the other two kids in the room.

Tommy couldn’t help but stare at this kid Zoomy. He was like someone from another planet. First of all, he was super small — sitting down, his feet didn’t even touch the floor. He had glasses that were so thick they looked like a part of a Halloween costume. When Zoomy was introduced, he began tapping his chin and didn’t stop. Everyone got silent for a moment, watching.

Ms. Hussey reached across the coffee table and touched Zoomy’s arm. “Got something to write on?” she asked.

Zoomy nodded, stopped tapping, and pulled a small notebook and a purple pen out of his pocket. Leaning close to his knee, he wrote several words.

Maybe he’s weird like Einstein, Tommy thought.

Next came Early, the girl. The first thing Tommy noticed was how tidy she was. A thick black braid clung to the back of her head like a giant caterpillar. Her jeans had a crease down the front and her sweatshirt looked as if it had been folded five minutes ago. And what was that lilac smell? Couldn’t be Mrs. Sharpe, who only smelled like grown-up perfume, and it wasn’t the other four — Ms. Hussey always smelled like vanilla, that spoonful just before it went into making French toast.

Next Tommy noticed that Early looked ready to run. She was sitting as straight as Calder’s N pentomino, back and shins parallel, eyes darting around as if she’d landed in a crowded fishbowl. Suddenly, in a flash, Tommy realized that it would be nice if he and Calder and Petra helped the other two feel comfortable. Plus, if he made the first gesture, maybe this new girl would like him. After all, it was dark in Mrs. Sharpe’s living room and his Krakatoa might be less visible. He wished he’d washed his hair that morning. Reaching quickly for a plate of cookies on the table, he passed them to Early.

“Stop, boy!” Mrs. Sharpe’s voice cut like a knife. Tommy froze and three of the cookies kept going, landing with a plop in Early’s lap.

Tommy heard her suck in her breath, surprised.

Ms. Hussey laughed, somehow making it all better, and bustled around, straightening Mrs. Sharpe’s blanket and passing paper napkins to the kids. The napkins each had a fancy crest on them — two lions standing up on either side of a red-and-black shield with a crown on the top and three silver Xs running down the center. Long words in some other language decorated a scroll beneath the shield.

Petra reached an open hand toward Early, who gratefully gave her two of the cookies and a cautious smile. Tommy wanted to kick himself. Oh, well, he thought. Since when have my plans for girls worked out? He slumped back into his seat. Petra passed the third cookie to Calder.

“They’ll be careful,” Ms. Hussey assured her old friend as she passed around heavy glasses of lemonade. “Now. Should we cut to the chase? I’m sure these kids are wondering why on earth we’ve called them all here this afternoon.”

Ms. Hussey, Tommy noted, was the only person he’d ever met who could be normal with Mrs. Sharpe.

All eyes were on the old woman, who closed hers for a moment as if to gather strength. “I have called you here because you are all rather extraordinary. Mentally,” she added, with a hawk-like glimmer. She paused, as if to let that sink in. “Each of you has done some detective work that the adults around you were incapable of doing. Three of you have already worked together. My hope is that the five of you will be able to rise to even greater heights.” She paused for a shaky sip. Ms. Hussey reached to help her with the glass, and didn’t seem to mind when Mrs. Sharpe forgot to say thank you.

The old woman continued, her voice quavering. “I had hoped that within the past week you five would have begun an investigation on your own. That you’d have asked for my help. But perhaps, at this stage, you’ve been too busy trying not to be what you are and disguising things as opposed to revealing them. It’s a pity that children mature in such predictable ways.”

Tommy had thought Mrs. Sharpe was going to call them disgusting when he’d heard that disg. She’d said the word slowly, like she’d wanted them to think that. Tommy popped upright and glanced at Petra, who also looked shocked and embarrassed. No pleasing some people, Tommy thought to himself. He felt sorry for Zoomy and Early, who’d never met Mrs. Sharpe before. If he were in their seats, he’d be scouting out the nearest exit.

Tommy tried to peer at Early out of the corner of his eye. Was she even breathing?

Mrs. Sharpe cleared her throat. “One of the thirteen pieces stolen from the Farmer is irreplaceable.” She paused. The room was now silent. She cupped her hands together, as if in prayer, then anchored them in the saggy web beneath her chin. Stones sparkled from between swollen knuckles and veins.

Her skin is worse than ours, Tommy thought, and he wondered if any of the others had noticed the same thing.

“I speak of an extraordinary work of art, and there’s no need to say which one. Quite truthfully, this crime has destroyed me, in part because I feel responsible, as one of the trustees of the museum. As trustees, we are the people in charge of the institution and its future. But for the past year, we have been dillydallying around and arguing about whether the place could be restored or whether the collection should go to Washington! Those of us who were in charge should … should …” Mrs. Sharpe’s cheeks were flushed, and she now stabbed the back of the couch with a closed fist, as if holding a dagger. Even Ms. Hussey jumped.

“This is a crime so heinous that it must be resolved, and quickly.” The old woman closed her eyes. “There’s no excuse for endangering this collection the way we did. The thirteen missing pieces belong to each one of us in this room and to — to — the future of humanity. To millions of people all over the world. And to the wonderful spirit who collected and displayed them for the public —” Mrs. Sharpe broke off, ran her tongue around her lips, and patted them with her napkin.

She’s not a normal old person, Tommy thought to himself. She’s more like a bloodthirsty dog that would give anything to bite down on a plump squirrel.

Mrs. Sharpe was speaking again. “Suffice it to say that I am horrified by the loss of every item that was taken that night. Three are by the incomparable Dutch master Rembrandt, and he is considered to be one of the greatest artists of all time. I confess, however, that the one painting I referred to earlier, The Concert, has haunted my dreams since the moment it was stolen. You all know it, of course.”

“Not me,” interrupted Zoomy, staring up at the wall over Mrs. Sharpe’s head. “But I know you mean the Vemmer painting.”

“That’s Ver-meer, emphasis on the second syllable,” Ms. Hussey said quickly, not giving Mrs. Sharpe a chance to respond.

Calder, Petra, and Tommy all grinned, and Tommy leaned toward Zoomy and whispered, “I like Vemmer better.” Ms. Hussey frowned at him.

Zoomy nodded and Tommy thought he caught the tiniest smile on Early’s face. Score, he thought happily.

“To tell you the truth,” Mrs. Sharpe went on, raising her voice, “I have my suspicions about who took the art but truly don’t care to know who the thieves were or are. It’s the thirteen objects — every single one — that I want. If all are returned safely, I shall die in peace.”

There was no response. Somehow, it wasn’t possible to tell Mrs. Sharpe that she shouldn’t think about dying — she’d probably think it was rude. Calder dug one hand into his pocket, making a sudden clacking sound.

“Thirteen and seventeen, the art and the date of the robbery. Both prime numbers,” he blurted. “And so is five. One of a kinds, if you know what I mean.”

“Mmm, well put, boy.” The old woman’s face creased into a shape that was almost friendly. “Oddly fitting. A prime crime.” She paused for a moment, as if waiting for someone to comment. No one did. “Not possible to divide by multiples,” the old woman finished curtly.

“Fewer red herrings.” Ms. Hussey nodded, and Mrs. Sharpe lifted her chin, as if to say, Exactly.

Their old teacher cleared her throat. Reaching for her computer, she gave the group a slideshow glimpse of the thirteen pieces, from the interactive FBI website. She passed the screen extra close to Zoomy.

First, the delicate, dreamy Vermeer … next, a ship filled with panicky faces … portraits of well-dressed people … a friendly self-portrait by Rembrandt … a scene of the country with a tall, skinny tower and a bridge … sketches of horses and dancers and musical instruments … a man writing at a table … then two objects, one a brass eagle from the top of a flagpole, the other a heavy-looking drinking cup.

The kids had seen some of the pictures in the news, but not all of them. Calder, Petra, and Tommy whispered comments to one another, partly because Zoomy and Early were listening.

“Jeez, how did they handle so much? I hope they didn’t put that eagle in a bag with the Vermeer!” Petra said.

“Or the cup in with the big landscape!” Tommy added. “That’d be a bad move!”

“Whoa, nasty storm: nightmare waves, even for a surfer dude. Those guys are saying, ‘Lemme outa here!’ ” Calder observed.

Mrs. Sharpe raised her hand for quiet. “Don’t assume everyone old is hard of hearing. Some of us are robins who never miss a worm. Let us hope that the five of you can stop trying to impress each other long enough to get to work.” Mrs. Sharpe dropped the last three words as if they were tissues she’d just used for blowing her nose.

Instantly the room was quieter than silent.

Wasn’t she just trying to impress us? Tommy wondered. He didn’t like adults who were nice one moment and nasty the next — or were unjust. After all, Zoomy and Early hadn’t said a thing.

Mrs. Sharpe now looked around at the group, who squirmed on their chairs. She sighed. “I wouldn’t ask you five to engage in this investigation if I didn’t believe that you will make an extraordinary, unexpected, and understated team. A team that no one will notice.”

Forgetting his Krakatoa, Tommy lifted his glass the tiniest bit in Calder’s direction, like an adult doing a toast after a speech. After all, how could no one notice them if they’d been invited by a rich grown-up to do something cool? It was a joke.

At that moment, Tommy’s lemonade slipped from his grip and descended in slow motion, the glass bouncing on the thick carpet beneath.

“Scaz!” he gasped.