Original: 매드 독
Choi Kang-Woo is a former detective, but he now works as a veteran insurance investigator. He leads a team that attempts to reveal insurance fraud cases.
In 2017, the percentage of insurance purchase rate in Korea is 95%. There’s only 5% chance of meeting someone with no insurance plan. Choi Kang Woo is a former detective who works as a private investigator after having his wife and son badly hurt in an airplane accident. Jang Ha Ri used to be a gymnast, but she now works as an investigator for Mag Dog. One day, Kang Woo and Ha Ri run into Min Joon in the midst of a fraud crime investigation.
Two years ago, Kang Woo’s team was investigating on an insurance fraud case that involves a man named Ko Jin Chul. While chasing him down, the team’s newbie gets in a car accident, which causes Kang Woo to miss his flight to Jeju Island that he planned to go with his wife and son. The plane carrying Kang Woo’s wife and son crashes. The pilot responsible for the crash turns out to be Min Joon’s brother, Kim Bum Joon.
Min Joon threatens Kang Woo to get what he wants with a clip that contains all the illegal deeds Kang Woo committed while investigating cases. Kang Woo secretly goes into Min Joon's house where he finds the articles related to the plane accident posted up on the walls.
Kang Woo finds out that Min Joon is after Lee Mi Ran who made Bum Joon take out 8 different insurance policies before the plane accident. Kang Woo helps Min Joon by chasing Lee Soo Oh to find out why Kim Min Joon is after Lee Mi Ran, who's supposed to be dead.
Both Kang Woo and Min Joon go after Lee Mi Ran who is still alive. Min Joon steals Cheetah's phone to get information, and he finds Lee Mi Ran before Kang Woo does. Meanwhile, Hong Joo realizes there's a dark secret regarding the flight accident that's thought to be a suicidal crash.
Kang Woo doesn’t understand why Ko Jin Chul killed Lee Mi Ran and why he had to hurt Min Joon. Mad Dog members set a plan to catch Ko Jin Chul and Min Joon moves on his own after he wakes up from the hospital. Meanwhile, Hyun Ki tells Ko Jin Chul to make sure Min Joon is dead.
Ha Ri, Noo Ri, and Soon Jung work together to find out who killed Ko Jin Chul at the police station. Kang Woo and Min Joon trade information to figure out the secret behind the crash and why Lee Mi Ran told Min Joon that Taeyang Insurance was involved in it. Meanwhile, Kang Woo returns as the manager of Claims Examination Team in Taeyang Insurance.
After Kang Woo became the manager of the Claims Examination Team again, he uses all his resources to trace Kim Bum Joon's original insurance policy. Kang Woo finds out that the dates Bum Joon took out the policies were fabricated to make it look like he crashed the plane as a suicide. Mad Dog members and Min Joon work together to find the original piece and figure out that Hyun Ki is keeping the file in his house.
Kang Woo is deeply saddened by the fact that Han Wu, who's been Kang Woo's friend for 22 years, betrayed him. Meanwhile, Min Joon approaches Hyun Ki and learns that the new evidence to solve the mystery of Flight 801 accident is with Chairman Cha.
Kang Woo successfully saves Min Joon, Ha Ri, and Soon Jung who fell in the trap that Chairman Cha and Hyun Ki have set, but Kang Woo gets caught. Hong Joo warns Kang Woo to stay low, but Kang Woo ignores it. Min Joon gets attacked by a man sent by Hong Joo, but he manages to survive. Meanwhile, Mad Dog members work on finding people whose claims were unpaid to reveal the truth behind Flight 801 accident.
Mad Dog members find out Noo Ri is Chief On's son. Chief On tells Noo Ri to lay low and hide somewhere. The prosecutors from Seoul Prosecutors' Office seize and search Mad Dog's office and confiscate everything, including the video of Kim Bum Joon's original insurance policy.
Noo Ri gets stabbed in place of Min Joon and loses consciousness. Chief On meets up with Hyun Ki to make a deal. Hong Joo plans to bring Hyun Ki down by working with Lee Young Ho. Meanwhile, Min Joon holds a press conference to reveal the original black box.
Both Kang Woo and Min Joon get arrested at the press conference for false accusation. Min Joon feels guilty, so he claims that everything was done by him. Meanwhile, Hyun Ki tells Hong Joo that her father, Chairman Cha, killed his father.
Kang Woo meets Hong Joo to show her that he has a recording of her making a deal with Chief On Joo Sik. When Hong Joo tells him that no one would believe his words, Kang Woo tells her that Noo Ri will be the one revealing the recording. Meanwhile, Hyun Ki holds a press conference to confess something shocking.
Hyun Ki does not stop at pushing over the responsibility of JH 801 flight crash to his dead father and Jung Pil, but he puts pressure on Kang Woo and Min Joon. The fight to catch Hyun Ki is set off by Mad Dog. Meanwhile, Young Ho tails after Mad Dog and no one knows exactly who he is after.
Kang Woo realizes that Young Ho has his eyes on Min Joon. Before the Mad Dog team gets a chance to reveal what Hong Joo and Hyun Ki did, they hold a press conference.
