After the forensic technicians had taken photos, the two of them put on gloves and shoe covers before entering the scene. There were eight beds in the narrow container house. The corpse had fallen to the ground facedown with blood flowing everywhere. The stench of blood filled their nostrils.

Chen Shi sniffed and smelled something sweet. He looked around and found an empty bottle under the bedside table.

This was a domestically manufactured carbonated beverage bottle, which had been emptied. Chen Shi unscrewed the bottle cap and found a strange pattern in the bottle cap. He glanced at the body of the bottle and took out identical bottles from the bag that the workers put empty bottles in. He unscrewed the bottles one by one.

He handed the bottle cap to Lin Qiupu and said, “Look. What’s this?”

Lin Qiupu looked at it in the light and said, “A black butterfly?”

“There is no prize redemption for this drink. This bottle cap is suspicious. It might be a signature left by the killer.”

“A new serial killer has appeared in Long'an again?” Lin Qiupu asked. The last thing he wanted to see was a situation like this.

Lin Qiupu went out with the drink bottle and asked the workers about it. A worker said that he had bought the drink but didn't notice anything about the bottle cap when he was drinking it. Later, he had put it on the table. The victim might have drunk from it as well.

The worker asked fearfully, “Was there poison in the drink?”

“Unlikely.” Lin Qiupu responded.

Chen Shi thought of the drink poisoning case ten years ago. Was this case done by the murderer's younger brother? Of course, this conjecture didn’t have any evidence backing it up.

The killer's technique was efficient and there were no signs of struggle at the scene. This time, it was highly likely that it would be a serial killer case.

Lin Qiupu called Chen Shi over to the car and asked, “Are you dating my sister?”

“How can you tell?” Chen Shi smiled.

“You think I didn’t notice her recent changes? I don't know what she likes about you, but this is her freedom!”

“Thank you, Captain Lin, for your approval.”

“Oh please. I have something here. Go back and take a look.” Lin Qiupu took out an envelope from the car and handed it to Chen Shi very seriously.

Chen Shi was about to open it and Lin Qiupu said, “Hey, it's important to handle the case first. Just look at it after you have headed back!”

The police officers visited the surrounding area. There were no security cameras here. The security guard uncle originally said that he had been on duty at the time, but when he realized the seriousness of the murder, he told the truth. He had received an anonymous call saying that his granddaughter had been hit by a car on her way to school. He couldn’t get through to his wife's phone. So, he went to the hospital to check out the situation. He was gone for about an hour and it turned out that nothing had happened to his granddaughter.

Lin Qiupu asked, “Who knows your mobile phone number?”

“Just relatives and friends. I haven't given it out to outsiders.”

Chen Shi said, “The murderer this time is very capable. The killer who seemed spontaneous in his actions had actually pre-arranged everything.”

“I didn’t ask you to come here to praise the murderer.”

“I'm not God. At the moment, I'm as clueless as you are.”

“I shouldn’t have called you to come!”

“But I think the drink is the key. Let’s go to the nearby kiosks and convenience stores and buy the same brand of drinks.”

“Go ahead! I'll look for witnesses again.”

Lin Qiupu, in order not to omit any details, questioned all the people around who could be questioned again. He also tried to find surveillance footage. At the same time, Chen Shi ran around all the convenience stores in the nearby streets, bought all the beverages of the same brand, and unscrewed all the caps off.

At half past eight, Chen Shi returned. Lin Qiupu was smoking gloomily. Looking at his expression, you could tell that he hadn’t made any progress.

Chen Shi opened his hand and there were three bottle caps with butterfly patterns in his hand. He said, “I found them at three convenience stores, and I asked their owners. Their supplier is the same person. I haven’t looked at the surveillance footage. You can send people over to check.”

“Do you mean that anyone who buys such a cap will die?”

“Doesn’t it sound familiar?”

“A beverage poisoning case ten years ago was like this. This time, the killer has upgraded it to become more difficult and even more cruel. Is the killer Wang Sunxu under the name of ‘Zhou Xiao’?”

“For now, it's quite possible!”

“Let’s investigate!”

Lin Qiupu asked someone to go to those three convenience stores to find their surveillance videos. He and Chen Shi found the beverage supplier. The person was an agent. He claimed that the nearby convenience stores and supermarkets were all his customers. They asked to check his inventory.

The agent took the two of them to a small warehouse, opened it, and there were cartons of drinks inside. There were about one hundred boxes of the carbonated drink.

Lin Qiupu had originally planned to buy them for inspection, but when he saw the quantity, he paused and asked, “Can they be opened?”

“No way. No one would want them afterwards.”

“I'll buy ten cases at the wholesale price!”

“Okay then.”

The agent moved over ten boxes. Lin Qiupu and Chen Shi opened the bottles together. Soon, all the caps were piled up at their feet. The caps were all normal. Chen Shi said, “It seems like he had tampered with them in the stores.” Then he took a sip from one of the drinks.

“If someone buys a drink with the bottle cap, he will die. Therefore, we should notify the manufacturer to stop them from shipping these to major retail outlets.”

“I'm afraid that notifying the manufacturer would be too late. All citizens should be notified not to buy this drink.”

“Okay, we’ll do that!” Lin Qiupu made calls immediately.

Chen Shi said, “What about these opened drinks?”

“Just get rid of them. Nobody will drink them now anyway.”

“I'll take a few bottles... No, I’ll take a box!”

“You sure aren’t polite.”

After they parted, Chen Shi went back first. When he got home, he opened the envelope entrusted to him by Lin Qiupu. It contained two copies of the same contract, which said “Things to Note While Dating Dongxue”. It listed in detail what Lin Dongxue hated eating and doing. It also mentioned how to deal with issues like having one party cheat on the other, unplanned pregnancies, and other unexpected situations.

Chen Shi was shocked for a full ten seconds. Was there really a brother in the world that would worry this much?

At the end of the contract, Chen Shi was required to sign, ink his fingerprints, and it stated that the contract was confidential. Chen Shi crumpled the two pieces of paper and threw them into the trash bin outside.

The next day, Chen Shi rushed to the office early. Lin Dongxue saw him and asked, “Is it true? The matter about the bottle cap murder?”

“It's just a guess at the moment. Has the case been filed formally yet?”

“Captain Lin has informed us to attend a meeting later, and more than half of the police officers in the team have joined the task force.”

“Let’s wait and see what he says at the meeting!”

At the nine o'clock meeting, Lin Qiupu explained the situation. Some of the police officers who didn’t know about the situation were shocked. Lin Qiupu had dark circles under his eyes and he had probably stayed up all night. He turned off the lights and put a few videos on the projector.

The videos clearly captured a man in a black sweatshirt walking into a convenience store and going to the shelf with the drinks. He quickly slid something out of his sleeve and tucked it into the shelf.

The same scenario was recorded in each of the three convenience stores where Chen Shi had bought the drinks with the tampered bottle caps. Lin Qiupu said, “The suspect did not show his face as he’s sly, but judging by his height and age, he is most probably Wang Sunxu.”

A policeman raised his hand and asked, “Anyone who buys a bottle cap with a butterfly pattern will be killed. How can the killer ascertain this?”