Chapter 1: The Moment the Key Turned in the Lock

Xu Yan stood in front of her apartment door, her feet heavy, her hand clutching a plastic bag filled with groceries. The April breeze carried a lingering chill, slanting through the dim, narrow corridor on the fourth floor of the aging building, making her shiver. But the wind wasn’t what sent a cold spike through her spine.

It was the key.

The one stuck in her door.

Her own key was still in her handbag, right where she’d placed it earlier. The key now protruding from her lock was not hers. More specifically, it was in her apartment door’s lock, yet she had never seen it before.

Xu Yan lived in an old residential complex in the southern outskirts of the city—a fading relic of the 1990s, built with red bricks and more memory than mortar. The place was populated mostly by elderly residents and a handful of long-timers reluctant to relocate. She had moved in just six months ago. The rent was low, the commute short, and until today, the most unsettling thing she’d encountered was a noisy cat that liked to scream at night.

But today was different.

Her heart thudded in her chest as she stared at the door. Rational thought told her she should call the police immediately. But there were no signs of forced entry. The lock wasn’t damaged, and the door hadn’t been tampered with. The key simply sat there in the lock, quiet, casual—as if someone had been in a hurry and forgotten to pull it out.

She hesitated. Then, with a breath held tight in her lungs, she reached out and pulled the key free. It was cold, metallic, unnervingly smooth in her palm. She inspected it closely. It looked almost identical to hers—but not quite. The notches on the blade were subtly different. Not wanting to overthink it, she inserted her own key, turned it slowly, and pushed the door open.

The apartment was silent.

Everything seemed untouched. The living room curtain hung half-drawn, just as she’d left it that morning. Slanted sunlight poured over the worn-out sofa, and the soft, familiar scent of jasmine—the fragrance oil she always used—lingered in the air. The doormat was in place. Her key pouch still lay where she’d tossed it on the shoe cabinet before rushing out to work.

But then her eyes landed on the sofa.

The throw pillows were out of alignment.

It may have seemed like nothing, but Xu Yan had always been particular about this. Every morning before leaving, she made sure the two cushions were perfectly straight, their seams lined up and facing the center of the room. Now, the one on the left had rotated slightly. It wasn’t the wind—none of the windows were open. And she didn’t have a cat.

Her eyes narrowed. She stepped lightly toward the kitchen.

Everything looked clean. The stove gleamed. The fridge door was closed. But then she saw it—sitting next to the sink.

A mug.

Not just any mug, but a ceramic one—blue-gray, matte, unfamiliar. Its rim was stained with a dark brown ring, as if someone had just finished a cup of coffee. She had never seen it before. She always used glass cups. And she lived alone. No guests had visited recently. She never let strangers into her home.

Xu Yan took a careful step back.

Her phone was in the pocket of her coat. She slowly reached for it and dialed 110.

“Emergency services. What’s your emergency?”

She kept her voice low and steady. “Someone’s been in my apartment. There was a key in the lock when I got home. It’s not mine. And there’s... there are things inside that don’t belong to me.”

“Are you safe right now?”

“I think so. I just got in. I didn’t hear anything unusual.”

The dispatcher took her address and instructed her not to touch anything suspicious. Officers would arrive shortly.

She ended the call and leaned against the kitchen doorway, eyes fixed on the mug, thoughts racing back over the past few days. She vaguely remembered hearing noises in the hallway at night—soft footsteps, perhaps, like someone pacing. At the time, she had assumed it was the neighbor’s cat. That morning, she’d heard someone call her name from downstairs. Quiet, but unmistakable.

It had been a man’s voice.

The police arrived within fifteen minutes. Two officers swept the apartment but found no one hiding, no signs of forced entry. The lock was intact. The windows secure. The mug and the unfamiliar key were collected as evidence. Xu Yan provided a detailed statement. The initial assessment? Possibly a prank. Maybe someone had copied her key. Since there was no sign of theft or harm, the case could only be filed for now.

That night, Xu Yan didn’t go home.

Instead, she checked into a nearby budget hotel, carrying nothing but a small overnight bag. She lay awake on the stiff white sheets, replaying the day’s events over and over—the key in the door, the misaligned pillow, the strange mug. Then, a memory surfaced: a month ago, she had bumped into a man in the stairwell, right at the corner.

He was wearing a property maintenance uniform. But his face had been unfamiliar.

She had apologized quickly and rushed off without thinking. Now, she couldn’t stop thinking about the coldness in his eyes.

The next day, she took the police report and went to the property management office to request security footage. But it was an old building, and surveillance cameras were few and far between. None had been installed near her unit.

She spoke to the manager, a bespectacled middle-aged man. After hearing her story, he frowned and said, “You mean the guy who came last week to help with maintenance? Last name was Li, I think. Only worked two days, then never showed up again.”

“Do you have his contact details?”

“You’d have to ask the main office. I don’t have them.”

When Xu Yan left the office, the sky had turned gray. Thick clouds hung low over the building. She stood beneath the tree outside and stared up at the entrance to her hallway. The once-familiar structure now felt foreign—ominous. She had the uncomfortable sensation of being watched, as though someone was observing her from the shadows.

She returned to the hotel and decided to change her locks.

By that evening, she had hired a locksmith. Following the police's suggestion, she replaced her entire lock system with a smart lock. She also installed a small security camera just inside the apartment door. Even if the hallway wasn’t monitored, at least she would know what happened inside.

That night, a new fear crept into her mind.

What if the owner of that mysterious key had entered her home not just once, but many times?

What if it had been happening for weeks—or longer?

She started to reevaluate her memory of the past few months. There had been small oddities—moments she had brushed off. The water heater’s settings mysteriously changing. A potted plant on the balcony slightly moved. A food wrapper in the trash for something she didn’t remember eating.

At the time, she’d chalked it up to forgetfulness or distraction. But now, each incident felt like a breadcrumb, leading her to a terrifying truth.

Someone had been inside her apartment while she was away.

Not once. Not twice. But repeatedly.

Xu Yan stopped turning off the lights at night. She placed a kitchen knife by her bed. She even rigged a motion sensor alarm behind her bedroom door. But the more precautions she took, the more anxious she became.

At 2 a.m., the roar of a motorcycle engine tore through the silence outside her window. She shot upright, heart pounding, eyes wide.

Then she heard it.

A soft, metallic click. Just outside the door.

It sounded exactly like a key slipping into a lock.

Then—silence.

Xu Yan leapt from the bed and crept to the entrance. Peering through the peephole, she saw nothing. The hallway was empty.

She checked her phone and replayed the footage from her newly installed camera.

Nothing.

But she had heard it. That faint but distinct sound. Not imagined. Not a dream.

Real.

And Xu Yan now knew, beyond any doubt—

This was just the beginning.