He was a disabled military officer who doted on his research work in the Northwest.

One evening in early December.

After school, the students gradually left the campus.

Gu Bei was on duty today, and he dawdled around the classroom with his broom in hand—he wasn't interested in sweeping, but was waiting for Shen Tang. Shen Tang had finished discussing something with Lu Yanzhi before leaving, even later than him.

He finished sweeping the floor, locked the classroom door, and walked towards the school gate with his backpack. As he passed the teaching and research offices on the first floor of the teaching building, he subconsciously glanced in that direction—

Then his steps suddenly stopped.

The door to the research office was ajar. The lights were off inside, but the light from the corridor shone through, revealing a figure bent over, fiddling with something in front of the safe.

Gu Bei's first reaction was—a thief?

He moved forward two steps, and could see more clearly from the crack in the door—

Not a thief.

It's Ma Decai.

Ma Decai was crouching in front of the safe, his back to the door. The safe door was already open, and he was pulling out a brown paper file folder from inside, his movements both hurried and careful, constantly glancing back at the door like a thief.

Gu Bei held his breath and silently took two steps back, pressing himself against the shadow of the corridor wall.

He saw Ma Decai stuff the file folder into his briefcase, then close and lock the safe, stand up, straighten his clothes, and walk out of the teaching and research office as if nothing had happened.

As Ma Decai left, he glanced at both ends of the corridor—

Gu Bei was short and pressed against the shadows in the corner of the wall, remaining completely still.

Ma Decai did not notice him.

The footsteps faded into the distance.

Gu Bei waited for a full minute to make sure the person was far away before stepping out of the shadows.

His heart was pounding.

Teaching and research office. Safe. File folder.

The city-wide mock exam papers are kept in the teaching and research office's safe.

He dashed out of the school gate.

……

The military compound. Mu Qinghan's courtyard.

Shen Tang had just returned from seeing patients at the hospital and was in the east wing packing her things for class the next day.

"Tangtang! Tangtang!"

Gu Bei's voice came from outside the courtyard gate—panting, as if he had run all the way there.

Shen Tang opened the door and saw Gu Bei leaning against the gate, panting heavily, his face flushed red.

"Why are you here? Why are you running?"

"I—I saw Ma Decai—" Gu Bei hunched over, panting heavily, and it took him several seconds to finish his sentence, "He stole the exam papers—in the research office—in the safe—I saw it with my own eyes—"

Shen Tang's expression froze instantly.

She pulled Gu Bei into the house, poured him a glass of water, and waited for him to tell her the whole story.

Are you sure it's a safe? Are you sure it's a file folder?

"I'm sure!" Gu Bei patted his chest confidently. "I recognize that safe—I saw it when the principal asked me to retrieve something from the research office last time. It's different from the usual metal safe used for locking documents; it has a combination lock. Ma Decai opened it with a key."

Shen Tang remained silent for a few seconds.

Ma Decai stole the mock exam papers from the entire city.

The purpose is self-evident—to leak exam questions to a class.

The exam papers were sealed and only opened before the exam. He looked at them and put them back; who knew someone had tampered with them in between? Even if the class's grades suddenly improved—that could be attributed to "my good teaching" or "the students' hard work." Who can prove it was a leak?

Unless—someone catches him red-handed stealing the exam papers.

But catching them in the act is too difficult.

"Let's go find Teacher Sun."

……

教师公寓

Shen Tang and Gu Bei found Sun Xiulan in the office.

Gu Bei recounted what he had seen. Sun Xiulan's face turned ashen after hearing it.

"Ma Decai!" She jumped up, her chair sliding backward and hitting the wall. "He actually dared to steal the exam papers—I'm going to the principal right now—"

"Teacher Sun, wait a moment." Shen Tang stopped her.

Sun Xiulan turned to look at her: "Wait for what? Can something like this wait?"

"If we report it now, what will he say?" Shen Tang's voice was calm. "He'll say—'I'm a teacher in the exam-setting team. Isn't it normal for me to go to the teaching and research office to check if there are any printing errors in the chemistry exam papers I set?' Gu Bei saw him take something out of the safe, but didn't see which specific exam paper he took—he could say he took his own chemistry exam paper, looked at it, and then put it back."

Sun Xiulan's anger was somewhat extinguished by these words.

She stood there, clenching her fists, and after a long pause, she managed to squeeze out a single word through gritted teeth: "Then what do you suggest we do?"

"And he's already seen it," Shen Tang continued her analysis. "He's memorized all the answers he needed to. If we don't do something, he won't need to take any more risks—he could simply tell the answers to Class One before the exam. We won't even have any evidence."

"Then should we look for Class One? Find evidence that he leaked the questions—no, Class One is in cahoots with him, they definitely won't say something like that." Gu Bei muttered to himself, dismissing his idea.

This really won't do; the class won't say anything. Are we just going to ignore it like this? Sun Xiulan's face grew even uglier.

"Unless—" Shen Tang changed the subject, looking at Sun Xiulan, "the version of the exam paper he reviewed is invalid."

Sun Xiulan was stunned for a moment.

"Ms. Sun, you are the head of the math teaching and research group, so revising and replacing test papers is within your authority, right?"

"Yes. I reviewed the final draft. If I found any problems, I could request a replacement of the individual question."

"Then tomorrow you should follow the formal teaching and research process and submit an application to the Academic Affairs Office—requesting that the last bonus question on the math exam be replaced because the 'bonus question lacks sufficient differentiation.'"

Sun Xiulan realized what was happening, her eyes lighting up: "The paper has been corrected—the one he looked at before is now invalid! If he wants to leak the questions, he'll have to steal it again!"

"right."

"Then we'll catch them in the act—"

"No." Shen Tang shook her head.

Sun Xiulan was stunned. "Why?"

"Because we don't have surveillance cameras," Shen Tang said calmly but confidently. "Ma Decai went into the research office alone, closed the door, and only he knows what happened inside. Even if we bring people to stake out the place, if we go in a minute earlier, he won't have made a move yet; if we go in a minute later, he'll already be out—the timing is crucial. If he becomes alert and backs down, we'll have no chance to catch him later."

Gu Bei, standing anxiously to the side, scratched his head and puffed out his cheeks: "This won't work, that won't work, what do we do?!"

Shen Tang glanced at him: "What's your rush?"

She turned to Sun Xiulan, her tone calm and composed, showing no sign of anxiety: "Teacher Sun, trust me. Tomorrow, just follow the procedures to change the questions—there's no need to deliberately spread the news; the academic affairs office will just notify the members of the question-setting team, and Ma Decai will naturally find out."

She paused for a moment.

"I'll bring you something tomorrow. Handle the new papers the way I tell you—you won't need to catch anyone in the act, you won't need witnesses; the evidence will speak for itself."

Sun Xiulan looked at her.

The eighteen-year-old girl sat on a student chair, her gaze unusually calm for her age. She didn't explain what the thing was, but the certainty in her eyes was convincing.

Moreover, given her meticulous nature, she must have been certain before she spoke.

Sun Xiulan was silent for two seconds, then nodded heavily. "Okay. I believe you."

"And—" Shen Tang continued, "Since the bonus question is being replaced, I have an idea for the new question."

As she spoke, she picked up a piece of draft paper and a pen from the table and quickly wrote down a set of mathematical formulas and derivations.

"Look at this problem. It's very difficult. Truly capable students can solve it on their own, but average people would give up as soon as they see it. The key is here—"

She circled the third step of the derivation, explaining, "In this step of the standard answer, the preconditions have been cleverly changed. The derivation process appears perfectly correct and logically consistent; most people wouldn't notice the problem."

Sun Xiulan leaned over for a look, her brows twitching slightly.

Shen Tang said, "But in reality, the conclusion of this step is wrong—only someone who truly derives from scratch will discover this flaw and work around it using their own method. Someone who just memorizes the answer will simply copy the standard answer verbatim, even including this error."

"One question, and the whole class got the exact same answer, even the wrong way they did it—" Sun Xiulan murmured.

"Then it's not a coincidence. It's a leak." Shen Tang put down her pen. "That thing can prove who touched the test paper, and this bonus question can prove who memorized the answers. Whether they stole the paper or memorized the answers—they can't escape."