Jang Jaegyeong is the Anhyun Police Station's Narcotics Unit's ace. One day, while pursuing drug dealers, Park Joonseo, a friend who hasn't been in contact for 20 years, comes to visit him. The following day, Joonseo's cold body is discovered, and Jaegyeong gets abducted and forced to become addicted to an unknown drug.

As Jaegyeong chases down those who abducted him, he finds out that Joonseo's body has been handed over to the family for a funeral without an autopsy. He rushes over to Joonseo's funeral, where he sees friends from 20 years ago gathered. Soon after, Joonseo's will gets read in front of everyone, stating the names of those who get to share his 50-million-dollar life insurance payout.

While everyone is in utter confusion after hearing the names of the beneficiaries in Joonseo's will, Won Jongsoo, Park Taejin, and the rest of the friends adamantly oppose an autopsy for Joonseo's body. However, at the request of Joonseo's wife, Choi Jiyeon, they proceed with the autopsy. Meanwhile, Jaegyeong and Oh Yoonjin search the house of Lee Myeonggook, the third name on Joonseo's will, to learn about his identity.

Jaegyeong suspects Kim Changsoo might have something to do with how he got abducted and drugged, and Yoonjin visits Jiyeon's house but sees someone unexpected. While investigating the area where Joonseo was found dead, he discovers a CCTV camera by a mill and finds a lead in the case.

The CCTV server, the only lead to the case, disappears in flames. Jaegyeong senses that someone is actively interfering with the case. Jaegyeong, Yoonjin, and Joosong track down the address of Audiophile, the company Joonseo had founded, and there, they meet Min Hyeonwoo, a high school student who knew Jaegyeong and Yoonjin would come.

Jaegyeong learns about the connection between the Lemon Mulberry drug and Joonseo's death, while Jongsoo proposes the development of a bioindustrial complex in Pilo-dong. At that time, Jaegyeong obtains the Lemon Mulberry dealer's contact and plans to set up a deal as a buyer while under disguise as a druggie.

Yoonjin makes her way to the director's office of the Woonjong Medical Center to investigate the medication error incident further. However, she faces an unexpected crisis. Meanwhile, Jaegyeong watches dashcam footage from a vehicle that was spotted on the day of Joonseo's death and freezes when he sees an unexpected person appear.

Jaegyeong puts the puzzle pieces together of the day Joonseo died, and a rift between Jongsoo, Taejin, and Jiyeon pushes them apart. Meanwhile, Jaegyeong, Yoonjin, and Joosong visit Hyeonwoo, discover the whole story between Joonseo and the medication incident, and then find a clue to the murder at the mill.

Jaegyeong knocks on Jeong Yoonho's door to arrest him for the murder of the mill owner. Meanwhile, Jaegyeong, Yoonjin, and Joosong gather at Jaegyeong's house to look over Myeonggook's work log. Joosong prepares a surprise birthday cake for Yoonjin, but an unexpected guest comes.

Taejin helps Yoonho stay in hiding. However, Yoonho is getting harder and harder to control. Meanwhile, Jaegyeong closes in on the Lemon Mulberry distribution group, Gong Jinwook, and chases down the person who had abducted and drugged him into addiction, leading him to the antique shop run by CEO Yoon.

With CEO Yoon dead and the only subordinate who could testify also dead, the case turns into another enigma. Meanwhile, Jaegyeong gets Yoonho's location through Kang Sijeong and heads to arrest him, and Yoonho, who is now at the point of no return, charges at someone in his last attempt to survive.

Jaegyeong narrows the investigation, while a rift forms between Jongsoo, Taejin, and Oh Chihyeon due to Yoonho's uncontrollability. Meanwhile, Jaegyeong and Yoonjin meet with Jeong Sangae, and he takes them to Youngryun Cold Storage, promising them he'll tell them the truth.

Jaegyeong, Yoonjin, and Joosong meet No Gyumin and Chae Gyeongtae's mother, and they hear the story behind the fire incident that happened 20 years ago. Meanwhile, Taejin fights back at Chairman Won and Jongsoo, who both want to abandon him, and Jaegyeong gets ready to arrest Jinwook in his attempt to flee the country.