Disappearance can be far crueler than death, and at the age of thirteen, Maria Park had little experience with either.

She ran her nails along the metal arm of the office chair, click-click-click, back and forth. Noelle, her older sister, clutched Maria's other hand protectively. She wanted to go home, but home was wherever their mother was, and nobody knew that.

Seated on the floor beside them, Vincent Stein stared down the hallway at the investigation team, his knees drawn up to his chest. The investigators were talking about his brother, Henry. Pointed questions echoed off the cold tile and bare walls of the Saturn Technologies building.

Vincent's presence, as welcome as it was, did little to reassure Maria. If anything, her best friend's nervousness amplified her own fears.

"Are they going to be done soon?" she asked. She hoped Henry wasn't still angry at Mom; they certainly shouted a lot that morning. Maria had told one of the federal agents about this earlier, and he'd smiled in a way that made her wish she hadn't mentioned it.

"Any minute now," said Noelle, maintaining the same stoic front that had allowed her to survive at the police academy. "You're making Vince nervous."

To unsettle Vincent was not difficult; he was a fidgety young man, one year Maria's senior, whose nails were gnawed to the quick. "They think Henry did something to her," he said. "They really think so."

"Let's not jump to conclusions, all right?" said Noelle. From what she understood, only a few people had accessed this floor of the building when their mother vanished. Five, specifically: her, her mother, her sister, and the Stein brothers, Henry and Vincent. Few people came to work on the weekend. If the worst had happened, Henry seemed like the most plausible culprit, but...

But the worst couldn't have happened. There was no body. Henry couldn't simply have erased their mother from existence. She had to be alive somewhere, but then, why didn't she respond to their texts?

"No," said Noelle, as much to reassure herself as to placate Maria, "they're not going to arrest Henry today. They might have more questions later."

"You think?" asked Maria.

"To help us find Mom."

Reassured somewhat by her sister's expert opinion, Maria ruminated quietly until Henry Stein's office door swung open. The federal agent who had been introduced to them as Ms. Singh stepped out, followed by Henry, who was downcast but not handcuffed. "If you think of anything," said Singh, "tell us. The longer she stays missing, the harder this gets. Forensics will take your prints and DNA while we talk with her brother. They worked together, I assume?"

Henry nodded and forced a crooked smile at Vincent and Maria before a lab tech led him away. Maria waved back, but Vincent only sighed. Recognizing their low spirits, Ms. Singh knelt down on one knee and addressed them both as Noelle watched.

"You've been very patient. I just need to talk to your Uncle Johann, and then you can all go home." She rummaged through her briefcase and produced two lollipops, which she gravely presented to Vincent and Maria. The gesture might have been received with more gratitude if they'd been a few years younger. As it stood, Maria glumly fiddled with the wrapper, but didn't open it, while Vincent rubbed and twirled the stick between his fingers.

"Um, what if you can't find her? What do we do?" asked Maria.

A slow shake of the head. A brittle smile. "That shouldn't concern you right now. That's what professionals are for."

"There were professionals on the Hindenburg," muttered Vincent, with an uncharacteristic bite of teenage sarcasm.

"Listen," said Ms. Singh. "I'm going to give you all my card. If you learn anything, let me know. I've got to go now."

Vincent nodded glumly and pocketed the card as Singh rose to full height, handed two more cards to Maria and Noelle, and walked away.

INTERACTION TUTORIAL: Five characters have been introduced in a short span: Maria and Noelle Park, their friends Vincent and Henry Stein, and Ms. Singh. In addition, Maria and Noelle's mother, Alice, has been mentioned.

Using the comment thread below labeled Character Intro Questions, try asking questions about any of these characters and I'll provide background details or clarifying information, as far as I can without spoiling the mystery

"It looks like they've finished with the scene," said Noelle. "I think we can get in, but stay quiet and don't draw unnecessary attention."

Her two charges nodded and rose to their feet. When they reached the machining room, Noelle inspected the broken door and thought back on the disappearance.

A few hours before, all four of them - Maria, Noelle, Henry, and Vincent - had seen her mother enter the machining room and lock the door behind her. At that point, she, Maria, and Vincent had decided to hang out in her mother and Uncle Johann's shared office across the hall. Meanwhile, Henry went to his own office to rewrite a paper.

Then, about an hour ago, Vincent smelled the smoke pouring out from from under the machining room door and frantically shouted for Henry.

"We all thought Mom was trapped in there," mused Noelle aloud. "But when Henry grabbed a crowbar and broke in, she wasn't there. If she left, I don't know how all three of us missed her. We kept our door open."

That was unsettling, but it was nothing compared to the second problem. Noelle and Maria had overheard the agents talking about the elevator records. The only ways to access the third floor of the building were the elevators and the emergency stairs, which had an alarm. Every time someone used the elevator, they needed to swipe their keycard.

And the keycard records were clear: nobody had used the elevator between the time Dr. Park entered the machining room and the time the police arrived. Furthermore, the cameras at the front desk captured no evidence of her leaving.

"It's a hard problem," said Noelle. Yes, that's what it was. Once they solved the problem, they'd find their mother, and all would be well. She just had to keep the kids calm, and put on a strong face. "Why don't we look around and see what we learn?"

She opened the machining room door and was greeted by the lingering aroma of smoke. The machining room held a hodgepodge of equipment and reagents. Cabinets of chemicals with obvious spill-marks lined the walls nearest the door, and a fabricator suite took up the entire back half of the room. Between the clutter, the fabricator, and the central table covered in power tools, there was little room to maneuver. Something faintly blue was splashed over the table and floor.

There were no doors but the one they entered by, and the window was closed tight. This was the room their mother had vanished from.

Okay, thought Noelle. I just need to consider every possibility, and investigate things closely.

TUTORIAL: This is an interactive segment. Comment with suggestions for our sleuths. What should they inspect in the room? Once a reader has suggested an action, I will reply in the comments and, if relevant, will unlock some of the locked narration below.

✓A Simple Answer? (unlocked by David F. Weisman)

A few possibilities flitted through Noelle's mind, but the first, and simplest, was the window. Upon examination, it was closed and latched from the inside, and opened over a straight drop to the ground below. No handholds were evident on the wall, and the ground itself was sloped steeply, making it difficult or impossible to set up a ladder. Not only that, the loose garden soil at the base of the wall would have held obvious footprints.

Unless someone pulled her mother up to the roof, and somehow closed and locked the window as well, it was hard to see how she could have left this way.

✓The Source of the Smoke (unlocked by Melanthe)

A little exploration and sniffing quickly revealed where the smoke had come from: the opening of one of the big fabricators was covered in ash. A warning on the display read "Emergency shutdown: overheat."

Inside, there were the remains of what may have been a flat wooden panel, badly charred. The lasers would certainly need a thorough cleaning, if they were recoverable at all. Noelle squinted at the panel more closely, and recognized it.

"I think it was supposed to be your birthday present," she said to Maria. "Don't worry... she can make another one when she comes back." Maria didn't respond; as sad as it may have been, the destruction of a present was the least of her concerns.

Maria and Noelle had amassed a sizable collection of laser-cut designs in wood, gifts from their mother. Sometimes, she would download pictures, turn up the contrast, and scan them. But when she felt a more personal touch was necessary, she used a stylus and tablet to guide the cutter manually, regularly checking her handiwork to get just the right depth and sharpness of line.

Noelle mused over this. Could the cutter have been left on, then overheated and set the wood on fire?

"But that means," said Noelle, "that she started to make this, but in the middle of her work..."

"What?" asked Vincent.

"I don't know. But something must have pulled her away from the station."

✓ A Belt? (Unlocked by strand VV)

Maria searched around for signs of a struggle. Was anything out of place? The cabinets were messy, but they'd always been that way. Uncle Johann was a clutterer, and her mother had long given up on doing anything about it.

If anything, the table in the middle of the room was slightly neater than usual. A metal belt occupied pride of place. Though it was still incomplete, with an open panel exposing its tangled innards, the attached clips and suspenders made it clear that it was meant to be worn. Three LEDs were visible in the front.

The absence of evidence of a fight had probably inflamed suspicions against Henry further. The culprit, the investigators had probably reasoned, must be someone whom the victim knew.

"But if they're going to accuse Henry," mused Noelle. "They need to explain how..."

Vincent and Maria looked at her quizzically, but she didn't finish the sentence aloud. Shutting her eyes, she inspected the floor.

"They might have tested with luminol. Maria, lights off, please."

Maria flicked the light switch. With the window shades down, the room was dark enough now to see a faint blue glow spread across several wide areas of the floor.

"Why's it glowing?" asked Maria, concerned. "Is it radioactive or something?"

"No. It's a test for blood." Seeing Maria's alarm, Noelle hurriedly added, "But it also gives wrong answers, too. Maybe a reagent in the lab set it off, or some cleaning agent."

Vincent bit his lip. "If it was blood, there was a lot of it."

A series of unwanted ideas flashed through Noelle's mind. There were power tools here. There was carbon cleaner, too, a big bucket of it. Acids for etching. And the fire could have been set by overloading the laser cutter deliberately, to cover up the smell. And the loud machining station could have covered up the sound of -

NO.

Noelle's voice quaked a little as she tried to drive the thoughts away: "No, no, it would take too long. Even with that sink, there's no way."

"What would take too long?" asked Vincent.

"Nothing." Collecting herself, Noelle shook her head fiercely.

✓A Less Messy Explanation (unlocked by strand VV)

She followed the spill marks along the cabinets, and soon found the culprit: bottles of antifreeze, at least some of which had been improperly capped. She then checked around the machining equipment, and saw the telltalle discoloration of leaked and spilled coolant. And if someone had mopped up these coolant spills and spread it all over the floor...

"The antifreeze has a glowing additive. Could've interfered with the luminol test."

She rose back to her feet.

"Mom's still alive," she said. "She must be."

Having finished investigating to the best of their ability, the three were preparing to leave when Maria snapped to attention.

"Hey! A mouse!" said Maria.

"What?" Sometimes, Noelle wondered if her sister had some sort of attentional disorder. She never sought a diagnosis, but...

Maria pointed to the hulking fabricator. "A mouse just ran behind there. It was wearing something... shiny? I don't know what."

Brow furrowing, Noelle crouched to peer under the machine. It was too dark to see and too loud to hear, but something was moving there. But why would a mouse be wearing anything? Was it some kind of lab specimen?

"Henry said they test devices on the mice," said Vincent, then added, noting Maria's look of worry, "It doesn't hurt them. But they don't do it in here. They test in the animal testing -"

The door swung open, interrupting these murine speculations, and Uncle Johann entered. "There you are. Er, you all shouldn't be in here." He didn't meet their gaze. "Let's get you somewhere safer. There's - well - there's - there's something I need to tell you."

It was then that Uncle Johann relayed the bad news. A thorough search of the surrounding area, including hospitals, buses, taxis, traffic cameras, and the morgue, had turned up not the slightest trace of Dr. Alice Palmstroem Park. It was as if she had vanished off the face of the earth.