Chapter 1: A New Neighbor

The lobby clock read five in the afternoon. A warm golden light streamed through the windows of the Esmeralda building, painting the worn tiles with a nostalgic glow. Leo and Lucía sat on the first-floor steps, watching elderly Teodora feed her cats from her half-open door. It was an afternoon like any other, until a noise broke the routine.
A moving truck pulled up in front of the main entrance.
"Another new tenant?" Lucía asked, leaning over the railing to get a better look.
"It seems so," Leo replied, frowning. "That apartment has been empty for years. Who moves into a place like that without notice?"
The truck driver got out and opened the back. From there, a tall, dark-haired man in sober clothing began unloading boxes. His movements were quick, almost mechanical, and he avoided eye contact with the curious passersby on the street.
"He's definitely not the kind of person you expect to see here," Lucía said, taking one of the cookies from her backpack.
"How's that?" Leo asked, amused.
"I don't know... He looks like one of those guys who appear in spy movies," she whispered as she leaned toward him.
Teodora, who had been watching silently, spoke from the doorway:
"That man has a rather unsettling air, don't you think?"
Leo and Lucía looked at each other, surprised to hear her say something so direct. Normally, Teodora was a source of kind stories and old-fashioned advice, but this time her tone held a hint of concern.
The man began to carry the boxes up the stairs, without waiting for help from the truck loaders. He walked past the boys without saying a word, but Lucía noticed something peculiar: he was wearing gloves despite the heat.
"Hi, welcome to the Esmeralda Building," Leo greeted with his usual enthusiasm.
The man merely nodded and continued up.
"Wow, how nice," Lucía joked, raising an eyebrow.
Leo, always eager to explore, quickly stood up.
"How about we do a little investigating? He's bound to have something interesting in those boxes."
"Don't even think about it," Lucía replied, grabbing his arm. "You know what Mom says: 'Stay out of your own way.'"
But while they argued, they couldn't take their eyes off the strange man. When he disappeared onto the top floor, Leo sighed with a mischievous smile.
"I'm just saying, if this guy turns out to be a spy, we could end up solving a mystery."
"Or getting into trouble," Lucía said, though a spark of curiosity glimmered in her eyes.
The afternoon passed, but the arrival of the new tenant left a feeling of unease in the air. It was as if something in the building had changed, though no one could say exactly what.
Lucía glanced up at the top floor one more time before entering the house.
"I don't know what's going on, but I have a bad feeling about this guy."
Leo smiled, ready for the challenge.
"Perfect. That makes it even more exciting."
The next morning, the Esmeralda building returned to its usual rhythm. The neighbors' footsteps echoed on the stairs, mingling with the smell of toast wafting from the Martínez family's apartment and the metallic sound of Teodora's shopping cart. However, the arrival of the new neighbor remained the topic of conversation.
Leo and Lucía, as always, were having breakfast in the kitchen, but both were distracted.
"What do you think was in those boxes?" Leo asked, biting into a piece of buttered bread.
"Old books, maybe. Or tools. Maybe nothing interesting," Lucía replied, though her tone betrayed that she was also intrigued.
Leo smiled.
"Let's find out."
"Leo!" Lucía protested, rolling her eyes. "We can't just walk into someone's apartment."
"What if we just watch?" he said, winking at her. "Aren't you curious?"
Lucía sighed, but before she could answer, a loud bang on the floor interrupted them.
"What was that?" Lucía asked, looking up at the ceiling.
"I think it came from the top floor," Leo replied, already getting up from his chair.
They both climbed the stairs carefully, trying not to attract attention. When they reached the top-floor landing, they found something strange: one of the new neighbor's boxes was outside the apartment, as if it had fallen during the move. It was open, and crumpled papers and a small jar filled with a dark red liquid were sticking out.
"This doesn't look like an accident," Lucía murmured, leaning over to take a closer look.
"What is that? Blood?" Leo asked, but before he could touch the jar, the apartment door burst open.
The mysterious man appeared in the doorway. His dark eyes flicked between them and the box, and for a moment he seemed to hesitate. Then he bent down, picked up the jar, and put it in his pocket without a word.
"Can I get you anything?" he asked gravely.

But without any kindness.
Lucía took a step back, but Leo maintained his composure.
"Nothing, we just heard a noise. We wanted to make sure everything was okay."
The man stared at them for several endless seconds before answering.
"Everything's fine. Now, if you'll excuse me..." he said, taking the box and closing the door with a loud bang.
Lucía grabbed Leo's arm and dragged him toward the stairs.
"What were you thinking? Trying to talk to him? After what we saw?" she reproached him in an urgent whisper.
"And what did you want me to do? Run away? That would have been more suspicious," Leo retorted.
"And what about the bottle? What do you think it was?" Lucía asked, crossing her arms.
"I don't know, but... it didn't seem normal."
They both remained silent as they went down the stairs, each deep in thought.
That same afternoon, while they were playing cards in the small park in front of the building, Teodora approached them with her cart full of cans of cat food.
"Have you seen anything strange today?" she asked quietly, as if afraid someone might overhear her.
Leo looked up, surprised.
"Why are you asking?"
Teodora leaned toward them, her eyes shining with a mixture of fear and urgency.
"Because that man isn't here by chance. There's something he's looking for in this building. Something he's been hiding for years."
Lucía swallowed, but before she could ask any more questions, Teodora turned and walked down the hallway, leaving them with more questions than answers.
Leo watched her disappear and then turned to Lucía.
"Now we really have to investigate."
Lucía sighed, knowing that, whether she liked it or not, they were about to get involved in something much bigger than they imagined.
Night had fallen over the Esmeralda building, and an eerie quiet had settled over the place. From her room, Lucía could hear the usual noises: the Martínez family's baby crying, Teodora's old radio that always played classical music, and the occasional footsteps in the hallway. But that night, there was something else.
A creaking sound.
Lucía sat up in bed, straining her ears. The sound wasn't coming from the hallway, but from the ceiling.
"What the...?" she muttered.
She quickly got up, putting a coat over her pajamas. She stepped out into the hallway just as Leo appeared from his own doorway, holding a flashlight and looking equally alert.
"You heard it too?" he asked quietly.
"Yes. It's coming from upstairs, isn't it?"
Leo nodded, and together they climbed the steps in silence. The lights in the top-floor hallway flickered dimly, as if the building itself shared her restlessness. When they reached the landing, they saw that the door to the mysterious man's apartment was ajar.
"This can't be normal," Lucía whispered.
"That's why we're going to find out," Leo replied, turning on his flashlight.
"What? Are you crazy? We can't go in there."
But Leo was already carefully pushing the door open, making sure it didn't squeak too much. Lucía followed him, whispering protests that he ignored.
The interior of the apartment was stranger than they imagined. There was barely any furniture: a mattress on the floor, a wooden table covered in messy papers, and a lamp that cast eerie shadows on the walls. The box they had seen that morning was now open on the table, and next to it was some kind of old notebook with yellowed pages.
Lucía approached slowly, her eyes scanning the place.
"Leo, I don't like this at all."
"Look at this," he said, pointing at the notebook.
He carefully opened the lid and discovered strange drawings and diagrams. There was a map that seemed to depict the Esmeralda building, but with details they didn't recognize: hidden passages, symbols marked in various rooms, and a circle around the basement.
"What does this mean?" Lucía asked, but before Leo could answer, a sound chilled their blood.
Footsteps.
The two turned their heads toward the door, where a shadow was projected on the wall. Without a second thought, Leo turned off the flashlight, and they both hid behind a closet.
The mysterious man entered the apartment, carrying what looked like a package wrapped in dark cloth. He walked straight to the table, leaving the package next to the open box. He muttered something to himself, words the boys couldn't understand. Then, he picked up the notebook and began flipping through the pages quickly, as if searching for something specific.
Lucía held her breath. She knew that if they were discovered, things wouldn't end well. Leo, for his part, watched intently, trying to pick up any details that might be useful.
Suddenly, the man closed the notebook with a loud bang and headed for the door, taking the package with him. They waited in silence until his footsteps faded into the hallway.
"What just happened?" Lucía whispered, emerging from her hiding place.
"I don't know, but this is getting weirder and weirder."

—Leo replied.
Before they could investigate further, they heard a noise coming from the table. As they approached, they saw that one of the papers had slipped to the floor. Lucía picked it up and, looking at it, her heart leapt.
It was a drawing of the basement door.
Leo looked at Lucía, and for the first time, they were both in complete agreement. Something really important was happening in the Esmeralda Building, and they had to find out what it was.