In the 10th episode of "In Your Radiant Season," the narrative resumes with a heartfelt conversation between Chan and Ha-ran at the hospital. He reassures her that he'll be just fine, and they bump into Su-jin, leading to an awkward interaction where her bracelet unexpectedly breaks. Afterward, they celebrate Chan's birthday with a cake and a thoughtful gift—a cardigan crafted by Ha-ran. She jokingly declares that she'll use his free trial for the rest of her life.
Elsewhere, Mr. Park and Na-na return to his cafe, and she realizes what has happened. Now that he knows, Na-na breaks down, admitting she's terrified. Meanwhile, Su-jin discovers that one of her charms is missing. At home, Ha-ran finds the charm in her bag. As for Chan, he continues to write his story on his tablet.
The next day, Ha-ran decides to finally clean out the attic where she used to sleep all the time. In the process, she finds an old letter from Hyeok-chan describing having a roommate named Chan. That evening, she goes on a date with Chan to a movie theater. He plans to tell her everything afterward and lets her know as much. While Chan is getting popcorn, a fire breaks out, and everyone is forced to evacuate. Chan searches for Ha-ran but his ear acts up, causing him to experience flashbacks from the day of the blast. They finally get to each other, but Ha-ran can tell that Chan isn't okay.
At home, Ha-ran opens up to her grandmother about how Chan keeps insisting he's okay even when he's not. She encourages him to just stay by his side and give him strength. The next day, Mr. Park turns up at the atelier and brings coffee for everyone. He tells Na-na he knows about her checkup that day and insists on going to the hospital with her.
Na-na continues to offload her work onto Ha-ran, which is how Ha-ran ends up with a mountain of work on her desk. At the same time, Ha-young goes to Na-na's office and ends up finding a set of notes reminding her about things. She realizes that Na-na has dementia and has decided to move to a senior citizen care center in a few months.
She tells her sisters and they see the contents of Na-na's locked cabinet. Inside, they find a letter in which Na-na tells them she's going to France to relax and asks them not to find her. There are also three wedding dresses for the three granddaughters which she made herself.
Ha-dam calls You-gyum and urges him to reveal Na-na's whereabouts. They rush to the hospital and embrace their grandmother warmly upon sight. Mr. Park shares with Ha-dam the extent of You-gyum's efforts. She pays a visit to thank him for everything. Tae-suk learns the news and rushes to the hospital, apologizing to Ha-young for not being aware of Na-na's condition.
After thorough testing, the doctor reveals that Na-na's condition is not dementia but hydrocephalus, an abnormal accumulation of cerebrospinal fluid. Although the symptoms are similar, it can be improved through surgery. Na-na is immediately admitted to the hospital but tells her family members to focus on their work while Mr. Park takes care of her.
Mr. Park also reaches out to Chan, who comes over to support Ha-ran. While he's getting her some food, Su-jin encounters Ha-ran and asks if Chan is her boyfriend. Na-na then speaks to Chan alone and urges him to be with Ha-ran for a long time.
Before leaving, Chan speaks to Su-jin and warns her to stop messing with Ha-ran. He asks her again about what happened the day of the accident, but she refuses to reveal anything. Later, he remembers that he had told Hyeok-chan the truth about talking to Ha-ran.
The next day, Ha-ran returns to the hospital to give back Su-jin's charm. When Chan realizes this, he rushes to the hospital but can't find her. In fact, Ha-ran stops by the cafe on her way to the airport. She goes up to Chan's room to return the cardigan he lent her. While there, she stumbles upon the notebook in which Chan has made notes about Hyeok-chan and his memories.
When Chan returns home, she's already gone, and he realizes what has happened. After an extensive search, he finds her at the cafe in the evening. She asks him if he enjoyed pretending to be Kang Hyeok-chan.
