In the fifth episode of The WONDERfools, Un-jeong is shocked to see Chae-ni resurrect from the dead. However, he soon passes out in her arms beneath the Northern Lights. Meanwhile, Chae-ni returns to the junkyard and finds Ro-bin unconscious there.
At the same time, Ho-ran mocks Pal-ho for injuring himself during his fight with Un-jeong. She points out that the defect in his arm is spreading and slowly rendering it useless, but Ju-ran steps in to calm the two down. Ho-ran recalls bonding with Un-jeong when they were younger.
Back in the present day, Chae-ni and Ro-bin bring Un-jeong home and put him to bed. While trying to undress him, they notice several wounds covering his body.
Later, Gyeon-un and Ro-bin go out to buy medicine for Un-jeong. The two discuss how Pal-ho is hunting down "defective variants" and killing them, leaving them worried about how they can protect themselves from future attacks.
Meanwhile, Jun-mo tells Jeon-bok about the incident at the junkyard and says that Chae-ni refuses to explain anything to him. He decides to investigate the matter himself to uncover the truth behind everything happening in Haeseong. While examining the junkyard, he discovers bloodstains and becomes suspicious.
Elsewhere, Chae-ni stays beside Un-jeong while he suffers from nightmares. She comforts him after hearing him call out for his mother in his sleep.
That same night, Jeon-bok notices Chae-ni sneaking out of the house. She tries to stop her, but Chae-ni leaves anyway and heads back to Un-jeong's apartment. Meanwhile, Un-jeong wakes up and notices that Chae-ni has left medicine and supplies for him.
The episode concludes with a sense of impending danger and uncertainty as to what will happen next in this intriguing story of human variation and the consequences of their existence.
The next day, Chae-ni, Gyeon-un, and Ro-bin awaited Un-jeong at City Hall. As soon as he arrived, he tried to leave, but the trio followed him. Chae-ni clung to Un-jeong and insisted that they work together as a team. Meanwhile, Ho-ran watched the four leave City Hall and grew jealous of Un-jeong's growing closeness to Chae-ni.
Un-jeong took the trio back to the secret room where he had previously trained them. He tried once again to help them trigger their powers so they could learn to control them properly. However, Ho-ran suddenly appeared and trapped Chae-ni, Gyeon-un, and Ro-bin inside hallucinations to distract Un-jeong. She questioned him about the Child of Eternity, but Un-jeong refused to join forces with her and managed to stop her from controlling the trio. Furious, Ho-ran left the room.
That night, Un-jeong finally revealed to Chae-ni about the Child of Eternity and explained that the child could never die. He claimed that Chae-ni now possessed the same ability and asked whether she had ever undergone a heart transplant as a child. Chae-ni dismissed the idea as absurd, but Un-jeong insisted that Jeon-bok was connected to the experiments conducted at Hawondo Lab.
After returning home, Chae-ni confronted Jeon-bok about the surgery. Meanwhile, Un-jeong returned to his apartment, continuing to think about the Child of Eternity. Jeon-bok finally revealed that after the fire at Hawondo Lab, the Child of Eternity suffered severe brain damage despite his heart continuing to beat. She claimed that the child eventually died and that his heart was transplanted into Chae-ni, who was terminally ill at the time.
Chae-ni searched through archived records and newspaper articles about the incident. She read about the orphan children who were experimented on and became deeply upset. Later, Un-jeong spotted Chae-ni outside and quietly followed her. Chae-ni blamed herself for everything that was currently happening. Meanwhile, Jeon-bok discussed the junkyard incident with Jun-mo.
The following day, Un-jeong worried about Chae-ni and asked Gyeon-un and Ro-bin where she was. Later that night, Chae-ni visited his apartment, where Un-jeong offered her something to eat. The two later walked to a convenience store together, and on the way back, Chae-ni talked about the heart transplant. Un-jeong reassured her and told her to stop blaming herself for things she couldn't control.
Meanwhile, Ro-bin delivered food to Chae-ni from Jeon-bok, which left her emotional. Chae-ni broke down crying while Jeon-bok also sobbed on her way back to the restaurant. Eventually, Chae-ni returned home but refused to forgive Jeon-bok. She told her grandmother that both of them must spend the rest of their lives repenting for what had happened.
Chae-ni and Ro-bin moved into Un-jeong's apartment without his permission. The following day, Un-jeong took the trio back to the secret location and once again helped them practice their powers. He gave Chae-ni caffeine in an attempt to raise her heart rate, which successfully helped her teleport to different places.
Chae-ni grabbed Un-jeong's hand and teleported with him to a beach, but she still struggled to fully control her powers. Later, Chae-ni left Un-jeong behind and returned to his apartment to prepare a surprise birthday party for him. Ro-bin and Gyeon-un also helped organize the celebration.
That night, Chae-ni went out to buy a cake for Un-jeong when he unexpectedly joined her. During their conversation, Chae-ni asked about his mother, and Un-jeong admitted that he didn't even know whether he truly had one. The two continued bonding, and Chae-ni reassured Un-jeong that she would always be by his side. However, Un-jeong suddenly betrayed her by injecting her with a drug. Chae-ni lost consciousness and was taken captive by Dr Ha's followers.
Later, Un-jeong told Dr Ha everything he knew about Chae-ni and her powers, explaining that her abilities were connected to her heart rate. However, he avoided mentioning the coffee despite betraying her, and the episode ended with Chae-ni being taken away.
