Episode 4 of "Trigger" resumes at the police station, precisely where our last scene left off, with Lee Do compelled to fire upon Won-seong to safeguard his fellow officers. The reporter who captured the harrowing incident on camera promptly submits his footage to his superiors, who are thoroughly impressed by this exclusive scoop and urge him to continue investigating the matter thoroughly.
Elsewhere, Gyu-jin catches wind of the shooting on the news, sparking in him the notion to procure a firearm. With swift keystrokes, he dives into the internet's depths, where the very first website that greets him boasts of offering guns at no cost.
Post-incident, Lee Do finds himself confined in an interrogation room, grilled by the disciplinary committee. They delve into his military past, highlighting how he had eliminated 99 targets, implying that he executed Won-seong merely to round up his tally to an even hundred. Just as tensions escalate, Chief Yoon storms into the room, halting the inquiry abruptly.
He commands Lee Do to accompany him, acknowledging his bravery in saving the officers—a fact unknown to the disciplinary committee. Chief Yoon scorns the committee as cowards, daring them to apprehend him first if they seek a scapegoat. Instead, he instructs Lee Do to return home and recuperate. Outside the police station, a crowd of reporters eagerly awaits, prompting Hyun-sik to escort Lee Do discreetly to his vehicle and drive him home.
At school, Yeong-dung, bullied into submission, is coerced into assaulting his friend Gyu-jin. Seeking him out at his desk where he diligently studies, Yeong-dung delivers a sudden punch and then orders Gyu-jin to bring his mother's ID to school the next day.
Consequently, Lee Do faces a three-month suspension from duty. As he prepares to leave the station, he inquires about U-tae's whereabouts with Officer Jang, but another officer intervenes, reminding him of his suspension. Hyun-sik steps in to save the day, feigning a need for the information and updating Lee Do that U-tae has been rescued. Furthermore, they reveal that the van's license plate matches the one Lee Do had encountered during his run-in with delinquents rummaging through delivery boxes.
Lee Do discovers that U-tae, freshly released from prison, has taken up menial tasks for Seok-ho, the notorious leader of the Glory Gang. Despite his attempts to reach Seok-ho by phone, his calls go unanswered. Determined to uncover the truth, Lee Do secures Seok-ho's whereabouts from Detective Seo and promptly sets out to confront him.
Confronting Seok-ho, Lee Do inquisitively probes whether he is entangled in the illicit gun trade. Seok-ho defiantly disputes this, producing evidence that his men rely solely on knives, swearing none of them have ventured out armed with firearms. Lee Do, unwavering in his mission, informs Seok-ho that he is actively compiling evidence related to the case, issuing a stern warning: if Seok-ho is found to have any involvement, there will be severe consequences.
Later, Lee Do ventures to Dongwol Industry, seeking insights into Moon-baek's background. His business card reveals Moon-baek's position as a technical sales manager at the company, yet Dongwol Industry's status as a defense contractor prohibits the disclosure of employee information. Fortunately, a timely call from Officer Jang furnishes Lee Do with Moon-baek's residential address. Without delay, Lee Do rushes to the apartment complex and dials Moon-baek's number, requesting a meeting.
Meanwhile, Jung-man and his cronies delve deeper into the gun trafficking mystery with the help of Mr. Kim. It transpires that Jung-man, devising a cunning plan, spared Mr. Kim's life and deceived Seok-ho. Mr. Kim unravels the intricate method by which the guns are smuggled: disguised as ordinary parcels, exploiting Koreans' reverence for the sanctity of others' packages. He reveals that he was recruited to distribute the firearms, receiving half of a truckload meant for others. Frustratingly, he lacks means to contact the masterminds who reached out solely through telegrams, and Seok-ho confiscated his phone.
In this tension-filled narrative, every revelation brings Lee Do closer to uncovering the truth behind the gun trafficking, while the schemes of Jung-man and Mr. Kim weave a web of deceit and danger.
The firearms remain securely confined within the storage room, elusive even to Seok-ho's diligent search. Currently, their sole viable course of action lies in gathering arms from the individuals listed on the address roster. Jung-man dispatches his men on a mission to amass every weapon mentioned on that list.
Elsewhere, Ms. Kyung-sook persists in her lonely vigil outside the corporate gates, convinced that this entity bears responsibility for her son's untimely demise. The CEO, indifferent to her pleas, cruises past her picket line with impunity.
Meanwhile, Gyu-jin's tormentors persist in their intimidation, insisting he surrender his mother's identification. Later that evening, a mysterious package arrives at his doorstep, revealing a firearm within.
Lee Do seeks out Moon-baek, seeking clarification regarding the gun and its ammunition. Moon-baek confesses his intent to use it for self-euthanasia, citing his terminal cancer diagnosis with a prognosis of six months or less. He bares his surgical scars, evidence of three arduous operations. Regarding the bullets, he recounts stumbling upon an online scam promising free firearms; upon receiving his package, the website vanished. As for the address list, though discarded, Moon-baek retains it in his memory. Lee Do urges him to jot down the addresses, among which is Ms. Kyung-sook's.
Upon glimpsing Kyung-sook's name, Lee Do dashes out, with Moon-baek in tow. En route, his attempts to reach her go unanswered. At that very moment, she confronts the CEO, who has callously ignored her since her son's passing. The CEO, in his typical hauteur, orders his driver to mow her down.
However, when she brandishes the gun, the driver flees, and the CEO's arrogance dissolves into desperate pleas for mercy. Kyung-sook pulls the trigger, ending the CEO's life. She then answers Lee Do's call, confessing her deed and acknowledging the impending consequences. She humbly requests Lee Do's permission to prepare one final meal for her son.
Kyung-sook returns home to await Lee Do's arrival. Tragically, U-tae and his cronies beat her there. They force entry and demand the gun. Lee Do's timely appearance interrupts the confrontation, and he swiftly disarms U-tae. The ensuing scuffle sees Lee Do effortlessly overpowering the boys. Outside, Moon-baek, finding the spectacle thrilling, assists Lee Do by subduing the remaining thugs in the getaway car.
As Trigger Episode 4 concludes, Lee Do and Moon-baek rush Kyung-sook to the hospital, her body bearing the brunt of her altercation with U-tae and his gang.
