Episode 5 of "Buried Hearts" unfolds with an intriguing encounter as Hui-cheol and Eun-nam stumble upon Dong-ju within the confines of a suave suit shop. Realizing their acquaintance, Dong-ju confesses to a harrowing loss of memory. Eun-nam, ever considerate, hands him a phone with a promise to reach out. However, Hui-cheol, with an unspoken agenda, promptly informs Gu-ho of Dong-ju's whereabouts.
Gu-ho, ever vigilant, relays this information to Yeom Jang-seon before rushing to the scene. He catches a glimpse of Dong-ju exiting the shop, only to have the moment interrupted when a saleswoman beckons him back inside. Elsewhere, Chairman Cha receives a stark diagnosis from his physicians – his brain is undergoing shrinkage, a condition fraught with gravity.
Gu-ho's colleague returns to search for Dong-ju within the shop's confines, only to find him vanished like a whisper in the wind. Coincidence weaves its tale as Dong-ju finds refuge in a nearby restaurant, where Chairman Cha happens to be dining alongside Do-su, the esteemed neurosurgeon and Cha's son-in-law. Dong-ju slips away unnoticed, but the restaurant owner, recognizing him, alerts Chairman Cha, who promptly dashes out in pursuit.
Meanwhile, Eun-nam's call pierces through Dong-ju's haze. She imparts his true identity and an address, urging him to meet her that very night. Dong-ju's memory flickers to life as the saleswoman's words echo – tales of a planned journey with his bride, whom the saleswoman speculates to be Eun-nam, and his subsequent request for a discreet exit.
By this juncture, Gu-ho has tracked Dong-ju down, finding him perched on a park bench, lost in thought. Yet again, their approach is thwarted by Chairman Cha's unexpected arrival. Cha's eyes widen in disbelief upon learning of Dong-ju's amnesia. But Dong-ju, sensing the suspicious glances exchanged by Gu-ho and his companions, makes a daring dash for freedom.
In a poignant flashback, Eun-nam is depicted in a harrowing moment of desperation, attempting to end her life as she is consumed by the belief that Deok-hui orchestrated Il-do to murder her father, Sun-ho. Despite Deok-hui's fervent denials, Eun-nam accuses her of banishing her to the United States out of fear that she had recalled this dark secret.
Switching to the present, the narrative finds Eun-nam inquiring about a phone from Deok-hui, a device she uses to communicate with Dong-ju. Deok-hui responds by stating that the phone is currently with Il-do, who promptly hands it over to Eun-nam without hesitation.
Meanwhile, it's Hui-cheol's birthday, and Chairman Cha has arranged a festive dinner in his honor. However, when Chairman Cha arrives home, he momentarily forgets the occasion, leading to an uncomfortable silence before the reason for Hui-cheol's presence dawns on him.
Elsewhere, Il-do confides in Jang-seon that Eun-nam has instructed Dong-ju to visit their residence later that evening. Unbeknownst to them, Dong-ju has already recognized Il-do as the individual who shot him while rescuing him from drowning years ago.
As the night unfolds, Dong-ju arrives at the Cha residence, greeted warmly by Eun-nam, who opens the gate for him. He introduces himself to the family as Seong Hyeon, a revelation that stuns Secretary Gong so much that she accidentally drops Hui-cheol's birthday cake.
Chairman Cha discreetly ushers Dong-ju aside for a private conversation. Before departing, Dong-ju returns Eun-nam's cell phone to her. Subsequently, Dong-ju, Chairman Cha, Il-do, and Do-su converge in Cha's office for a discussion. Dong-ju reveals that fragments of his lost memories are slowly returning. In response, Chairman Cha urges Dong-ju to seek immediate medical attention at a hospital, to which Dong-ju reluctantly agrees.
Later, Deok-hui enquires after Secretary Gong, casually wondering if the mention of Seong Hyeon stirred any emotion within her. Gong responds matter-of-factly, noting the commonality of the name, which Deok-hui acknowledges, adding that "that child" is deceased.
In the hospital's hallowed halls, Yeom Jang-seon pays Dong-ju a visit, only to be met with immediate suspicion from Dong-ju. Shortly thereafter, Chairman Cha and Dr. Kang, Dong-ju's personal physician as well as Cha's, arrive on the scene. Dr. Kang senses something awry and promptly dismisses both men, citing Dong-ju's limited visitation privileges. He then informs Dong-ju of an upcoming psychiatric evaluation. Meanwhile, Gu-ho collects Jang-seon, revealing that he had previously planted a listening device in Dong-ju's hospital room, allowing them to eavesdrop on every conversation.
Elsewhere, Eun-nam ventures to Dong-ju's abode and discovers a photo of him alongside Sister Agnes. Intrigued, Eun-nam seeks out Sister Agnes at the church, relaying the recent developments. Sister Agnes shares a poignant revelation: Dong-ju had suffered a loss of memory once before, during his childhood. She also appears to recognize the name Seong Hyeon, sparking Eun-nam's belief that a visit from Sister Agnes might prove beneficial for Dong-ju.
Back at the hospital, Dr. Kang notices Dong-ju's bullet wounds but is met with a blank stare; Dong-ju claims no recollection of how he acquired them. A hospital staff member approaches to escort Dong-ju for testing, but in a twist, he turns out to be one of Gu-ho's operatives. He administers a sedative to Dong-ju and spirits him away from the hospital. Fortunately, Dr. Kang's instincts kick in, alerting him to the异样.
Dong-ju awakens to find himself in Yeom Jang-seon's sinister torture chamber. Jang-seon's henchmen subject Dong-ju to waterboarding, while Jang-seon himself mocks Dong-ju's pretended amnesia, urging him to cease the charade.
