On the 30th of June, 1997, a mysterious father-and-daughter duo arrived at Yuanlongli. They take the elevator to the abandoned 18th floor. When the security guards arrive, they find only the strange traces left behind by the two of them.

Ran Fangxu insists on investigating the route of the disappearance on the 18th floor, while He Yuanhang believes that directly searching for the missing people would be more efficient. The disagreement escalates, and the two go their separate ways.

He Yuanhang receives a message that someone wants to provide information about the missing father and daughter in person. The informant, Su Yuzhi, says the missing girl is her coworker, Xu Meng.

Ran Fangxu discovers the relatives of the Xu family. He decides to split up with He Yuanhang to visit them separately.

A theft occurs at the mine, and the precinct is setting up a task force. Ran Fangxu is also assigned to it. However, Ran Fangxu insists on continuing the investigation into the yet-to-be-filed missing persons case of Xu Meng and her father, and even assesses that it may be a homicide.

Wu Shanlong arrives at Nanshan police station. He Yuanhang and Ran Fangxu beat around the bush to make Wu Shanlong expose himself. Both of them deduce that Song Qingsong ordered Wu Shanlong to hire a hitman to get rid of Xu Meng.

Ran Fangxu and He Xiaohe run into the property manager, Qian Dajun, in the underground parking garage. Qian Dajun claims he is fixing the wiring and carries away an electrician's tool bag. Ran Fangxu uses the Tremaux algorithm to sweep the building and brings He Xiaohe up to the rooftop, where they run into Qian Dajun again.

He Yuanghang and Ran Fangxu found Su Yuzhi. She tells them that Boss Sun's name is Sun Zhengyong, who once tried to keep Xu Meng as his mistress. He Yuanhang and Ran Fangxu split up to find clues about Sun Zhengyong.

After opening the elevator doors, they see handprints and footprints left inside the lift, along with Xu Meng's handwritten note. They speculate how the father and daughter escaped from the elevator. He Yuanhang and Ran Fangxu return to the police team, where Captain Feng gives them high praise.

Ran Fangxu visits He Yuanhang's home to have a meal. He is warmly welcomed He Yuanhang asks Ran Fangxu to help him hide the fact that he's returning to the station.

He Xiaohe takes He Yuanhang to a psychiatric clinic. The doctor recommends long-term psychological counselling, but He Yuanhang dismisses the doctor as a fraud and refuses to undergo treatment.

He Xiaohe learns that there is an illegal internet cafe in the underground garage of Yuanlongli. The two of them go there, but the cafe owner and the children inside have never seen Han Ziang. However, someone has seen two of the four missing boys.

Ran Fanxu runs into Wu Shanlong. During the chase, Wu Shanlong panics and falls into a deep pit full of steel bars that pierce through his body.

Officer He keeps thinking that this case is being resolved too easily. So, he decides to investigate it in secret. He finds out about Xu Meng's ex-boyfriend, Wan Qiming. He decides to leave town to look for Wan Qiming.

He Yuanhang notices Xiaohe is discouraged, so he takes out Ran's old book. When Xiaohe opens it and sees the words "Fantian Theorem" written across the page, something stirs in her. She takes the book to the principal's office and says she needs the principal's help to catch the culprit.

Ran and the others track things to a small convenience store outside the illegal internet cafe. At first, the owner lies and claims that he doesn't know Xiaohe. But Ran senses that something is off and circles back. The owner has no choice but to admit that he has seen Xiaohe. By then, however, Xiaohe has already left the internet cafe.

After Xiaohe is rescued, she finds out that the underground internet cafe has been shut down. She is worried about the four young men who are into esports. She visits them at the detention center with Ran.

Ran remembers that Wu Shanlong was hit on the back of his head too. He deduces that the culprit might be the same person. Song Qingsong is probably back in town.

Xiaohe learns that He Yuanhang is no longer hallucinating Ran and realizes he has recovered. She then hands over Ran's belongings to He Yuanhang, telling him that he had collapsed back then after seeing those items.

Ran's messages make He realize that he must have found a major clue that can overturn their previous conclusion on the case. He suddenly recalls that in 2002, Ran mentioned the blue leather shoes in case 630 to him...

While Ran is playfully interacting with Xiaohe, something Xiaohe says suddenly clicks for him. Apart from the blue heels, there is another important clue that has been overlooked, which is the Bowls dress.

He believes that Ran must have uncovered something in Yuanlongli. He has to withstand the pressure from the demolition team and hold Yuanlongli at all costs.

Qian Dajun gives the police a full and detailed account of how he found the hand bones. He initially assumes that, after Qian Dajun's arrest, the demolition work at Yuanlongli will be put on hold. Unexpectedly, Jin Manlang is appointed as the new person in charge of the demolition.

He exposes Qiao Suqing in front of everyone. Initially, Yao Xincheng purchases Unit 415 under the Yao Baiyuan's name, and later entrusts Qiao Suqing to sell it. However, in 2003, Qiao Suqing re-purchases the property under Yao Xincheng's name.

Qiao Suqing's narrative appears airtight, but a forensic report on tissue traces from the blue heels ends up undermining her confession. Just as He and Xiaohe are at a loss, they suddenly discover a common link among several of the death scenes...

Based on the evidence in the wooden toolbox, the police successfully put Qiao Suqing in detention right before she could step out of the police station.

Xu Meng's interrogation officially begins. Under everyone's intense questioning, she openly confesses to all her crimes.

Faced with everyone's anger, the murderer shows no remorse and stands her ground. She believes Ran is the one who causes Qiao Suqing to betray her, and thinks she should have killed Ran long ago.