In Episode 5 of Yumi's Cells Season 3, we're introduced to the intriguing character of Soon-rok, a man operating on low-power mode. As he navigates a team dinner, his lack of responsiveness earns him the label of "cold" from his peers. However, when Kim Ju-ho orders him to leave the book street, allowing Yumi and Soon-rok to be alone, a spark ignites within Soon-rok's mind.
As Ju-ho's behavior continues to irritate Soon-rok, his Emotion Cell fuels the fire, making it grow bigger and bigger. This leads to the dramatic moment after the team dinner, when Soon-rok lies to Ju-ho and tells him that he and Yumi are watching a movie together. Despite his attempts, Soon-rok's Reason Cell struggles to stop the Emotion Cell from adding more fuel to the fire.
As they argue, Soon-rok texts Yumi, asking if she'd like to watch the movie with him. Reason Cell notes that this behavior is highly atypical for Soon-rok. Unfortunately, Ju-ho shows up at the theater as Yumi arrives. Yumi's Emotion Cell also sparks up, assuming that Ju-ho asked Soon-rok to call her.
Neither Yumi nor Soon-rok enjoys the movie. Afterwards, Yumi confronts Ju-ho directly, asking if he likes her. He confirms it, to which she clarifies that she doesn't feel the same way. Believing that Soon-rok was part of the scheme, Yumi gives him a piece of her mind and then leaves.
On her way back, it starts raining, and Yumi takes shelter near a department store. Soon-rok soon arrives, realizing that she would need an umbrella but learning that the store is out of them. Yumi apologizes for her earlier behavior, but Soon-rok doesn't accept it. He tells her she was cool.
Soon-rok rushes off to find an umbrella at another store. In his head, Reason Cell activates emergency power and informs the others that Soon-rok's behavior around Yumi has been unusual. It's broken two of his principles - to only watch a movie with someone he likes and to never judge a writer based on anything other than their work.
Soon-rok discovers a last remaining umbrella in a store and returns to give it to Yumi, explaining that he must return to Kim Ju-ho. As he begins to walk away, Yumi follows, insisting on sharing the umbrella until they reach the hotel. Despite Soon-rok's strict adherence to his rules, which prohibit him from sharing an umbrella with her, there seems to be no other option. On the way, he even pulls her away from oncoming traffic, reminiscent of a dramatic scene from a movie. At one point, Yumi notices a wallpaper of a Maltese dog on his phone and brings it up in conversation. He reveals that he has a Maltese named Kongsoon, which surprises Yumi. Despite his steadfast opinions about Maltese dogs, he loves Kongsoon just the same.
At the hotel, Soon-rok secures another umbrella and returns to the movie theater. There, Ju-ho confronts him and questions why he lied about Yumi already being at the theater when she arrived after Ju-ho. Soon-rok's cells assess that they cannot be honest in this situation and he chooses to remain silent.
In the morning, Yumi meets Dae-young and learns that Ju-ho returned to Seoul for an early interview and that Soon-rok is down with a cold. Yumi assumes it's because of the rain and drops by his room. She's surprised to learn that Soon-rok is no longer on Ju-ho's team. Yumi grows angry with Ju-ho once again. When she heads out for another sightseeing trip, another writer tells her that Ju-ho was complaining about Soon-rok to another writer before he left. Yumi then calls Dae-young and confirms that Ju-ho refused to work with Soon-rok in the morning. Yumi confesses that something happened with Ju-ho but doesn't divulge the details. Since she feels bad for Soon-rok, Dae-young suggests putting Soon-rok back on Yumi's team. She agrees.
Yumi tries calling Soon-rok after this but he doesn't pick up. When they return to the hotel, Yumi learns that Soon-rok went to the hospital. She visits him at the clinic and finds him asleep with an IV in his arm. She checks his temperature and he wakes up. The action triggers another spark in Soon-rok's head and Reason Cell quickly snuffs it out. Reason Cell brings an X-ray of Yumi's rib cage to Love Cell, who is described as a skilled surgeon. Love Cell finds a small heart in the X-ray and realizes that Yumi has feelings for Soon-rok. However, Soon-rok never dates his colleagues.
Once Soon-rok is discharged, Yumi treats him to a meal and tells him about the possibility of working on her team again. Love Cell suggests cutting off Yumi so that her feelings don't grow any more. So, when Soon-rok gets a call from PD Jang, he accepts her offer to go on a blind date. He even asks Yumi if she knows someone, clearly drawing a line between them.
Afterwards, Love Cell realizes that there were two X-rays placed on top of each other. Another heart is visible in the additional X-ray - Soon-rok's X-ray. At that moment, Yumi messages him and says he will get along well with one of her juniors. As she asks him if he wants to go on a blind date with her, Soon-rok turns and looks towards Yumi's room with a sense of understanding and appreciation for her feelings towards him.
