none of your business.

When Yu Yan knocked on Fan Jinci's office door for the third time, a loud crash came from inside, as if some glassware had been smashed against the door.

"I told you not to bother me!" Fan Jinci's voice came through the door, even more hoarse than when they last met.

Yu Yan glanced at his watch—11:18 PM. The corridor of the forensic center was deserted, with only the green light from the emergency exit casting an eerie shadow overhead. He turned the doorknob, and as expected, it locked.

"Chief forensic pathologist, I have a new discovery," Yu Yan said in a low voice, "regarding the 'Morning Star' stun gun."

Silence fell inside. Then came footsteps and the sound of a key turning. The door opened a crack, revealing Fan Jinci's pale face. A fresh bloodstain ran below his right eye, as if he had been grazed by something sharp.

"You'd better not waste my time," he said coldly, but still stepped aside.

The office was even more cluttered than before. The whiteboard was covered with new photos and notes, and several takeout boxes sat untouched on the coffee table. Most striking was the expensive mass spectrometer humming in the corner.

Fan Jinci walked with an awkward gait, his right hand hanging unnaturally at his side. He went straight back to his workbench and continued observing something under the microscope, completely ignoring Yu Yan's presence.

"I checked the records of all veterans who were discharged between 2004 and 2006."

Yu Yan walked to the workbench and noticed a small section of skin protruding from Fan Jinci's right wrist through the cuff of his pajama shirt—there was a strange scar there, shaped like a fallen butterfly with its wings slightly spread, as if it had been carved with the finest scalpel.

Fan Jinci keenly noticed his gaze and abruptly pulled down her sleeve to cover her wrist: "Get to the point."

Yu Yan withdrew his gaze: "We've identified three suspicious individuals, two of whom have alibis. The third is Zhou Lin, a former special forces medic who was discharged in 2005 for abusing prisoners of war." He took out a file from his briefcase. "Interestingly, he started a high-end medical equipment company last year, specializing in importing surplus supplies from the US military."

Fan Jinci's fingers paused for a moment: "The source of the stun gun?"

"Very likely. Even more coincidentally..." Yu Yan paused deliberately, "His coffee shop is right across from the shop where the suspect we arrested worked."

Fan Jinci finally looked up from the microscope, a hint of interest flashing in his gray-blue eyes. Under the light, Yu Yan noticed that the dark circles under his eyes were more pronounced, and his lips were chapped from dehydration. The scalpel lay beside him, its blade stained with some kind of pale red liquid.

"What are you analyzing?" Yu Yan couldn't help but ask.

Fan Jinci used tweezers to pick up an extremely thin tissue sample: "The substance from under the deceased's fingernails. It's not his." He suddenly stood up, but staggered due to dizziness, his right hand instinctively bracing itself on the table, the butterfly scar once again exposed to the light.

Yu Yan rushed forward to support him: "How long has it been since you slept?"

"None of your business." Fan Jinci broke free, but his strength was much weaker than before. "This organization has a special preservative ingredient, military grade, often used for..." His voice suddenly became softer, and his body swayed.

Yu Yan caught his sliding body with lightning speed, his hand landing precisely on the butterfly scar. Fan Jinci trembled violently as if electrocuted, weakly but resolutely pushing him away: "Don't touch there!"

The two stood frozen for a second, then Fan Jinci suddenly grabbed a scalpel and pointed it at Yu Yan: "I told you, don't touch me."

The scalpel tip trembled. Yu Yan slowly raised his hands: "Okay, I won't touch you. But you need to rest, otherwise you'll be the next one lying on the dissection table."

Fan Jinci sneered and staggered toward the sofa, but his knees buckled halfway there. This time, Yu Yan didn't dare touch him directly. Instead, he grabbed a pillow and placed it under Fan Jinci before retreating to a safe distance.

"Zhou Lin's company is in the West City Industrial Zone," Yu Yan said calmly. "I plan to go investigate first thing tomorrow morning."

Fan Jinci sat on the floor, leaning against the sofa, the scalpel still in his hand: "Foolish. If he's the murderer, going there is just suicide."

"So you need to give me some professional advice." Yu Yan squatted down to be at eye level with him. "For example, how do you identify a serial killer?"

Fan Jinci's eyes were half-closed, his long eyelashes casting shadows on his face: "He would...collect loot. Not necessarily common teeth or hair..." His voice trailed off, "...maybe some kind of gland..."

"Like an adrenal gland?"

"Hmm..." Fan Jinci's head slowly tilted to one side, "He also...monitors police operations..."

Yu Yan noticed that his fingers finally loosened their grip on the scalpel. He carefully removed the murder weapon and then gently helped Fan Jinci onto the sofa. This time, the other man did not resist and fell into a deep sleep almost instantly.

The butterfly scar was visible again. Yu Yan resisted the urge to touch it, and simply pulled the blanket over him. While adjusting the blanket, he noticed a perfectly symmetrical butterfly scar on the inside of Fan Jinci's left wrist—two lifelike butterflies in flight.

What kind of trauma could leave such an exquisite yet cruel mark?

The next morning, Yu Yan drove alone to the western industrial zone. Sunlight shone through the smog onto the gray buildings. Zhou Lin's company occupied an inconspicuous four-story building with a sign that read "Lin Dong Medical Equipment Co., Ltd." hanging at the entrance.

Yu Yan didn't notify his team, only bringing his service pistol and handcuffs. As he entered, the young woman at the reception desk gave him a professional smile: "Hello, do you have an appointment?"

"Criminal Investigation Team, routine check." Yu Yan flashed his badge. "We recently received a report of a stolen batch of medical equipment and need to check our inventory."

The girl's smile froze: "I...I need to ask the manager..."

"No need," a gentle male voice came from the stairs. "It's our honor to have Captain Yu grace us with his presence."

Zhou Lin looked to be in his early forties, tall and slender, dressed in a well-tailored gray suit, resembling a university professor rather than a retired soldier. He walked slowly downstairs, leaning on an ebony cane in his right hand, his left leg seemingly somewhat unsteady.

"Does President Zhou know me?" Yu Yan squinted.

"The city's youngest criminal investigation captain, who doesn't know him?" Zhou Lin smiled and extended his hand. "But I'm even more familiar with the one you invited out of retirement. We're... colleagues."

Yu Yan didn't shake that hand: "Fan Jinci, oh? You two know each other?"

"Indirectly." Zhou Lin withdrew his hand, a subtle flicker in his eyes. "When I was at military medical university, I had the privilege of reading his papers. His insights were brilliant, especially regarding the effects of post-traumatic stress disorder on the nervous system..." He gestured invitingly, "Would you like to take a tour of our warehouse?"

The warehouse was neatly arranged with various medical equipment, most of which had export labels. Zhou Lin walked ahead, leaning on his cane, occasionally offering a brief explanation. Yu Yan noticed that his left leg limped unnaturally, as if it were deliberate.

"I heard you imported a batch of surplus US military supplies?" Yu Yan asked casually.

Zhou Lin stopped in front of a row of metal cabinets. "Yes, mainly expired battlefield first aid kits." He opened a drawer. "They all have legal documentation. Would you like to take a look?"

Yu Yan glanced at them; they were indeed all ordinary medical supplies. His gaze was drawn to a small freezer in the corner: "Is that also imported?"

"Ah, that's specifically for storing biological samples." Zhou Lin's smile remained unchanged. "It requires a constant temperature environment of minus forty degrees Celsius. Want to take a look?"

Intuition kicked in Yu Yan's head. He casually approached the freezer, his hand resting on his sidearm: "Is it convenient to open it?"

"Of course." Zhou Lin pressed the tip of his cane, producing a soft "click," but a password is required.

Just as he bent down to enter the password, Yu Yan noticed a strange tattoo on the back of his neck—a butterfly with outstretched wings, strikingly similar to the scar on Fan Jinci's wrist, only in the opposite direction, as if it were rising rather than falling.

The freezer door slowly opened, and a cloud of cold air rushed out. Inside, a dozen or so glass containers were neatly arranged, each labeled. Yu Yan leaned closer to look and saw that the labels didn't have numbers, but rather dates.

"What is this?" he asked, pointing to the nearest container.

Zhou Lin's voice suddenly changed, carrying a morbid pleasure: "April 15, 2018, that was the first imperfect work." His cane suddenly pointed to another container, "But this... September 28, 2023, is almost perfect, only lacking the final step of sublimation..."

Yu Yan suddenly drew his pistol: "Zhou Lin, you're under arrest!"

Zhou Lin laughed, a sharp metal spike suddenly popping out from the tip of his cane: "What a pity, Captain Yu, you shouldn't have come alone." He whistled, and three burly men wielding sharp blades immediately emerged from the shadows of the warehouse. "Especially... without forensic pathologist Fan."

Just as the blade was about to pierce Yu Yan's back, the warehouse door was kicked open.

"Put down your weapon!" Fan Jinci's voice pierced the air like an ice blade. He stood at the door, his white coat fluttering behind him, and in his hand he held not a scalpel, but a police pistol.

Zhou Lin's expression instantly turned fervent: "Brother Fan! You've finally arrived!" His cane trembled with excitement. "Look, I've completed most of it! Just one last step... just your approval..."

Fan Jinci's face was paler than a corpse. His right hand, holding the gun, remained motionless, while his left sleeve trembled slightly, revealing the faint butterfly scar: "Zhou Lin, the game is over."

"The end?" Zhou Lin laughed maniacally. "This is just the beginning! You think you can get rid of me by cutting your own carpal artery? Look at the butterflies on your wrist, they'll never fly! Just like back in the lab—"

A shot rang out.

A burst of blood erupted from Zhou Lin's right shoulder, and his cane clattered to the ground. Fan Jinci's gun, smoke rising from its muzzle, his eyes as cold as icy ice: "The next shot will be to the head. Choose."

As the sirens echoed through the industrial area, Yu Yan realized he had been holding his breath. Fan Jinci was still holding his gun, but his eyes were unfocused, and his body swayed slightly. Yu Yan rushed forward and caught him before he fell to the ground.

This time, Fan Jinci did not push him away.

"Butterfly..." Yu Yan asked softly, his fingers gently tracing the scar on the other's wrist.

Fan Jinci closed his eyes, his voice so soft it was almost inaudible: "...That's another story."