Episode 5 of "Good Boy" rekindles our acquaintance with the academy days, where Dong-ju finds himself irresistibly drawn to the captivating beauty of Han-na. His distraction leads to an unfortunate encounter with a football, which sails straight into his face, leaving him sprawled on the track—a scene reminiscent of last week's cliffhanger, as Dong-ju lies supine, his gaze locked on Han-na above.
At the precinct, Ju-yeong faces interrogation from officers eager to uncover his role in the string of murders. He's undeniably a key player, yet he guards his secrets diligently, revealing nothing. Meanwhile, the hit-and-run vehicle has been discovered, and DNA analysis is underway. Ju-yeong remains unperturbed, content to observe how the evidence unfolds, whereas Dong-ju, overconfident, believes they've already nailed their suspect, convinced victory is merely a matter of time. Ju-yeong, however, harbors a deeper conviction—this is merely the prelude.
Dong-ju notices Ju-yeong continually glancing at his watch, a sign of something impending. As it turns out, Ju-yeong's connections are extensive; soon, he's joined by the overbearing prosecutor Seok-hyeon, who, invoking the Customs Act, forces the officers to surrender Ju-yeong into their custody.
As Ju-yeong prepares to depart, Han-ma mentions the watches and their link to the Golden Bunny group and Gyeong-il, piquing Ju-yeong's curiosity but failing to crack his composure. However, Dong-ju, unable to contain himself, coughs up blood and collapses outside, necessitating emergency hospital care.
In Dong-ju's absence, the team finds itself under severe scrutiny back at the office. The prosecution team descends upon them like a storm, clearing out the place and seizing everything in their path. It's at this moment that Man-sik and the others grasp the magnitude of their adversary and the elusive nature of Ju-yeong.
Jong-hyeon confides in Han-na, warning that Ju-yeong appears to be casting a suspicious eye on her. He fears that the situation could escalate and urges her to reassign to a different case for her safety. Yet, Han-na is undeterred, seeing this as a golden opportunity to act as decoy and draw Ju-yeong into their net.
At the prosecution office, Seok-hyeon dismisses the smuggling allegations against Ju-yeong, branding them as mere "trivialities" punished with mere slaps on the wrist. Ju-yeong faces a three-month salary deduction and a transfer from Insung Customs. Learning of this, Man-sik is incensed and resolves to delve deeper, determined to uncover a chink in Ju-yeong's armor. Regrettably, their search yields little. Ju-yeong leads a solitary existence, save for a slight acquaintance with fellow customs officer, Kim Yu-na, and harbors a past marked by the tragic accident that claimed his family when he was young. Hailing from Insung, he was raised by his uncle.
Regarding his finances, Jae-hong uncovers evidence of a rental deposit amounting to 120 million won, savings of 56 million won, and a modest subcompact vehicle. Nevertheless, Ju-yeong displays a meticulous nature, leveraging his Customs position to appropriate high-profile cars for personal use or disposing of them when scrutiny intensifies.
Han-na braves a confrontation with Ju-yeong by the dockside, presenting him with a folder, subtly taunting him with her sharpshooting metaphors. These documents feature a symbol—a blue sun descending over waters—that traces back to Cheong-il Shipping. Ju-yeong, who once oversaw this operation and holds sway as an Insung Customs officer, is clearly entangled. Han-na's gesture is both a strategic move to signal their closing in on him and a daring step that places her squarely in the crosshairs for retaliation.
Regarding the team, they discover assistance in the most unexpected of quarters. They enlist the aid of Dae-yong, who is convinced that the clue is embedded within a synthetic drug known as Candy, cleverly disguised in candy wrappers. Dae-yong is aware that the individual behind the creation of this substance is none other than the infamous Drug Demon. She is utterly deranged, yet unfortunately, Ju-yeong's connections run deep within Drug Demon's criminal empire. These ties also stretch back to Russia, albeit the team is still lacking substantial information on this front.
Drug Demon is fully prepared for the police's arrival, swiftly eradicating all traces of her drug den as the officers descend en masse. Dong-ju makes an appearance and manages to thwart a couple of thugs from a rival gang who were intent on terminating Drug Demon. However, this distraction leads to the entire gang becoming entangled in the ensuing explosion. Dong-ju leaps out of the window, clutching Drug Demon, thereby preventing her from meeting a grim fate. Everyone else emerges unscathed, but Ju-yeong has already outmaneuvered them. This is particularly pertinent since they are unaware that Drug Demon is the very girl they "rescued" from the gangsters, and she is placed under protective custody for the meantime.
She develops a fondness for Dong-ju, viewing him as a knight in shining armor, and grows envious of his affection for Han-na. The following morning, after more playful antics between Han-na and Dong-ju in the bathroom, the duo attends the police ceremony where Pan-yeol is delivering a rousing speech. Amidst the scorching heat, Dong-ju does his utmost to assist Han-na.
Later, Dong-ju procures some beverages, but unfortunately, just as Han-na appears to be developing feelings for him, Dong-ju collapses at the very moment they share a kiss.
