Episode 4 of "The Tale of Lady Ok" kicks off with So-hye and her men on an urgent quest to locate Gu-deok. Meanwhile, Tae-young finds herself in hiding, while Yun-gyeom ventures out to investigate the commotion. He swiftly connects the dots and reassures Tae-young of his discretion, pledging not to divulge her secrets. To foster her trust, he confides in her a profound personal truth: his inability to embrace a woman. They realize that their respective plights could be mutually resolved through marriage, an arrangement Tae-young reluctantly embraces.
Seung-hwi and Man-seok, driven by curiosity, attend the wedding and are utterly astonished to witness Yun-gyeom's presence. The Yuhyangso, also known as Mother's Sanctuary, suffers a humiliating blow and vows to bring the Seungs to their knees. Unbeknownst to them, Yun-gyeom harbors another secret: he is the clandestine leader of the Devoted Hearts Association, concealed behind the veil of a gisaeng house. The association's members, including the children he mentors, bear a tattoo of the 'heart' character on their shoulders. His latest endeavor is to rescue a child named Moon-soo.
Elsewhere, Minister Bak stumbles upon intelligence about gold deposits in Mt. Unbong, located in Cheongsu. He ropes in Scholar Lee Chung-il to oversee the illicit mining operations. However, Lee's wife, Madam Kim, harbors deep-seated distrust towards Bak, who had earlier orchestrated a frame-up against her husband, leading to their exile to Cheongsu. She turns down Bak's offer, but Lee secretly strikes a deal with Madam Hong to finance slave labor. In return, she seeks to wed their children, sweetening the pot with a lavish dowry. Madam Kim, swayed by the proposal, agrees to the betrothal.
In a different corner of the narrative, Tae-young's legal career blossoms, and she fosters a friendship with Yun-gyeom's younger sibling, Do-gyeom, while Seung dotes on her affectionately. She discovers that Slaver Ji has duped unsuspecting, illiterate individuals into selling their children to him. Tae-young steps in to lend a helping hand. It transpires that Ji and Lee are exploiting these slave children as miners, with Moon-soo among them. Moon-soo, a noble offspring who was ensnared for crossdressing and sold into a brothel, now finds himself trapped in this cruel reality.
At her home, she discovers Yun-gyeom undergoing a transformation and catches a glimpse of his tattoo. Amidst other pressing concerns, she informs Magistrate Seung about the children's plight. He, too, receives an anonymous tip (orchestrated by Yun-gyeom) revealing the slavers' route. Seung, Tae-young, and their officers intercept Slaver Ji's latest consignment of kids, leading to a fierce skirmish. Tae-young narrowly escapes injury as a masked Yun-gyeom jumps into the melee. His mask briefly slips, but he flees with Moon-soo after the slavers are apprehended. As for Slaver Ji, he seizes Moon-soo's friend as a hostage and makes a daring escape.
In a confrontation later, Tae-young finally confronts Yun-gyeom, who is visibly worried about her safety. She bombards him with questions, unable to fathom why he is risking his life so recklessly. He clarifies that the Devoted Hearts Association rescues outcasts like himself. He saved Moon-soo because he was the only slave child without a refuge. His mission is to train these kids in self-defense.
That fateful night, the sons of nobles visit the gisaeng house. One of them tries to make advances towards Hae-gang, Yun-gyeom's trainee, and is astounded to learn that Hae-gang possesses an effeminate demeanor. They attempt to murder him, but Tae-young intervenes swiftly. The nobles insist that Hae-gang is unnatural and pressure Magistrate Seung to take stringent action.
Tae-young promptly alerts Yun-gyeom, who is eager to rescue Hae-gang. She tries to advise caution, but he reacts angrily towards her. He promptly apologizes and explains that Hae-gang was the first child he had ever saved.
During the trial, Tae-young reveals that the King himself had exiled a bipolar individual rather than executing him. Magistrate Seung agrees to seek royal counsel, to the fury of the nobles.
Yun-gyeom thanks Tae-young as Hae-gang is bailed out. He understands that she is worried for him and offers to shut his operation. She knows he doesn’t wish it and so refuses.
Things go back to normal till Tae-young comes across one of the former slave kids who tells her about Mt. Unbong. It cuts to the remaining kids mining at the mountain. Lee and Minister Bak get news that Magistrate Seung is going to search the mountain courtesy of Tae-young. Moon-soo’s friend suddenly lets it slip that he wishes he could have escaped to the gisaeng house to train.
Bak assumes that the gisaengs are hiding rebels and Lee gets the idea of distracting Seung by rounding them up. They alert the provincial governor who orders Seung to find the rebels and the mountain search is paused. Lee finds Hae-gang at the Devoted Hearts hideout and kills him. His tattoo is revealed and Seung orders every man to be searched.
Tae-young tries to help Yun-gyeom escape who is furious by the developments. Seung happens upon them and assumes his son is innocent. Yun-gyeom can’t take it anymore and reveals everything. A furious Seung orders him to shut it down but Yun-gyeom refuses. Seung disowns him and tells him to leave.
Nearby, Man-seok is setting up a gig for Seung-hwi. The guards corner them with swords and they instantly take off their clothes, marking them safe.
As Yun-gyeom and Tae-young try to run, they are cornered in a square. Yun-gyeom hides in an inn but is soon dragged out. He complies and as Tae-young looks in horror, he doesn’t have the tattoo. He winks and turns out he is Seung-hwi. A flashback shows Yun-gyeom coincidentally hiding in Seung-hwi’s room. Seung-hwi understands the situation and offers to switch places for Tae-young’s sake.
Currently, Yun-gyeom and Tae-young feel immense gratitude, yet as they depart, Seung-hwi's demeanor turns melancholic. Yun-gyeom, moved by Tae-young's offer to clarify everything to Seung, vows to return one day. During the climactic scene of Episode 4 of The Tale of Lady Ok, Seung, with a heart heavy with remorse, confesses to Tae-young his regrets for his harsh paternal words. He pleadingly urges her to depart, convinced that he will bear the blame alone, but Tae-young stands firm, refusing to abandon him. At this critical juncture, the provincial governor arrives bearing a royal proclamation, declaring that Seung's negligence constitutes treason. While Lee receives accolades, Seung is stripped of his title and possessions, overwhelming him to the point of fainting. In an instant, everyone rushes to his side, the scene filled with tension and empathy.
