Kim Min-ji, daughter of mild-mannered "Manager Kim," has been bullied for a long time by a wealthy classmate, Ju Hye-ri, daughter of construction-company president Ju Kang-chan. Min-ji finally retaliates and beats Hye-ri up after enduring repeated abuse. Hye-ri's parents storm the school demanding an apology, and because Ju Kang-chan has money, prosecutors, lawmakers, and lawyers in his pocket, Kim Bu-jang caves and kneels in front of the family to beg forgiveness. Min-ji is humiliated watching her father grovel and lashes out at him afterward, accusing him of being a nobody with no status, and storms off saying she'll stay at her friend Hye-ryeong's house instead.

On her way there, Hye-ri and her crew ambush Min-ji. A fight breaks out; Hye-ri's boyfriend Seong Min-ho beats Min-ji badly, and when Hye-ri hits her in the head with a rock, Min-ji is knocked unconscious and appears dead. Panicking, the group calls in a fixer; Hye-ri identifies herself as Ju Kang-chan's daughter to get "cleanup" help, and they're connected to a gang, the Stray Dogs (들개파), led by Gye Du-sik.

When Min-ji doesn't come home, Kim finds blood and Hye-ri's hair tie at the scene. He tracks down the gang member chain: first intimidating Min-ho into naming the "fixer" Oh Min-cheol, then confronting Oh Min-cheol directly, interrupting him in the middle of trying to sell another man's daughter to a trafficking ring. Kim beats him down and, enraged by Min-cheol hiding behind his status as a legal minor, shoots him. Kim gets Min-cheol's phone to trace Min-ji's location, but instead receives a message that Min-ji is already dead. Distraught, he threatens the gang at knifepoint and gets arrested when police arrive, but while in custody, a call comes in from Min-ji's phone. Kim breaks free of his handcuffs and answers, only for the call to cut off before he can learn anything.

The government's Special Missions Bureau, led by Director Gang Guk-cheol, is tracking Kim by the codename "66," revealing he's a former black-ops agent. Kim enlists his old friend Seong Han-su, a taekwondo gym owner who owes him a debt, and together they fight off Gang Guk-cheol's men while tracing Min-ji's phone signal to a high school where local gang leader Shin Seong-ho is holding a meeting. There, Kim discovers Min-ji's phone charm in Shin's pocket. Shin had it because his crew scavenges lost phones to resell, not because he was involved in her disappearance. Kim tortures a confession out of him and learns the ugly truth that Min-ji's "best friend" Hye-ryeong had actually been using and mocking her all along.

Following the phone's trail through a homeless man who'd found Min-ji's discarded belongings, Kim and Han-su learn the getaway car was a white BMW and get a description of the driver.

Meanwhile, at the docks, Gye Du-sik's men stuff the unconscious Min-ji into a sack and lock her in a freezer warehouse, planning to use her survival (and the fact that she witnessed Hye-ri's assault) as blackmail leverage against Ju Kang-chan, who had once had Gye Du-sik's teeth pulled out as punishment. Min-ji wakes up inside the sack, terrified but alive. As Kim races toward the docks, evading Gang Guk-cheol, whom Han-su holds off in a dramatic bridge fight. Min-ji bangs on the freezer door screaming to be let out. The door opens... but it isn't her father. It's Gye Du-sik himself, and a brutal confrontation follows involving Du-sik's adopted enforcer Gang-min, betrayals among the gang members, and multiple deaths, while Kim searches the docks trying to reach her in time.

In one gut-wrenching moment, Min-ji actually sees her father and calls out, but, not realizing he's her rescuer and fearing he's part of the gang, she runs from him in the rain, flags down a passing car for help, and gets in... only to discover it belongs to Ju Kang-chan's people. She's taken to Ju Kang-chan's mansion, cleaned up, and brought before him. He interrogates her gently at first about her relationship with Hye-ri, but when he senses she may remember what really happened, he drops the act entirely and orders his man to kill her to protect his daughter. Just as Min-ji is about to be injected with poison, she cries out for her father, and at that exact moment, Gang Guk-cheol's team arrives at the mansion, forcing their way in.

Elsewhere, Nam Siljang (Ju Kang-chan's enforcer) kills Gye Du-sik for turning on his boss again, but not before Du-sik reveals with his dying breath that Min-ji is still alive, a fact that puts Ju Kang-chan and his daughter in serious danger of exposure.

With Min-ji's location narrowed down, Kim confronts Ju Kang-chan directly, calling out his cover-up, the kidnapping, and his ties to organized crime, framing it explicitly as one father to another. Ju Kang-chan, cornered, drops his polished-executive mask, hurling insults about Kim's poverty and worthlessness as a provider, and dares Kim to hit him, threatening to ruin both him and Min-ji using his political connections if he does.

Kim recalls his late wife Lim Yoo-jin's dying wish, that he protect their daughter no matter what, at any cost, and tells Ju Kang-chan that being a father is harder than being a breadwinner. When Ju Kang-chan swings first, Kim beats him down without restraint, breaks his leg, and declares that from now on he'll do anything necessary to protect Min-ji, morality aside. He then calls in a favor from his old contact Jo Pyeong-gyeon to have Ju Kang-chan's construction company financially destroyed, which happens, buying it out at a fraction of its value and ruining the family. Kim also reveals his old covert identity as North-infiltration agent "Code 66" in the process.

Min-ji, having watched all this, finally understands the depth of her father's love and the sacrifices behind his quiet, unimpressive exterior, and reconciles with him, regretting how she misjudged him.

Later, Kim's old comrade Park Jin-cheol tells Min-ji the full truth about her father's past as a legendary black-ops operative, encouraging her to appreciate him, noting that whatever the official record says, to her father she is his most precious achievement. With Ju Kang-chan and Hye-ri's family financially ruined, the arc closes, and the story transitions into Kim taking a job at Baekho Manpower Agency, which is where the episodic, case-of-the-week structure begins.