Episode 1 of "In Your Radiant Season" begins with Chan, a serious and reserved individual, working in a lab in Boston while speaking to Song Ha-ran on the phone. She's cheerful and upbeat, while he's all business. He sends her a pen engraved with her name, and she later tells him that she won a design contest with it. She describes how much she loves the scent of rain, and he draws a cartoon girl who sneezes in the rain.
Back in Seoul, we learn that Chan is the younger brother of Ha-ran's boss at work, Eun-chan. The team is about to enjoy a four-day holiday. Ha-ran, seeing a plane with a vapour trail in the sky, gets inspired and decides to visit Chan. She sends him a video message saying she's coming and tells him that she loves him. However, Chan only sees this message when she's already landed in Boston and is on her way to his university. He rushes towards a set of blue doors but is thrown back by a massive blast. Ha-ran arrives at the university to see firefighters and police everywhere. She gets a call from Eun-chan, who tells her that Chan didn't make it.
The story skips seven years. We see a fashion shoot taking place at Nana Atelier, the brainchild of a South Korean fashion queen named Kim Na-na. She's also the grandmother of Ha-ran, who is now working at the atelier as a leading designer. However, she's not the same cheerful Ha-ran anymore. She's distant and withdrawn, disappearing off to brood on her own.
Meanwhile, Seonu Chan arrives in South Korea along with his colleague named Ben. He has burn marks on his arm but is otherwise a changed man - a happy, always-smiling person who greets everyone with hugs. He and Ben are here for a work project but Chan is delighted to go sightseeing in his home city.
Elsewhere, Ha-ran does some doodling at her regular cafe. The cafe owner gives her an invitation to a painting exhibition and tells her to check it out. Chan ends up going there as well and is shocked when he sees Ha-ran. Outside the art gallery, he notices that some construction material is about to fall on her and instinctively calls out her name. It saves her, but Chan runs away before she can see who called her.
Chan and Ben attend their work meeting, which unexpectedly turns out to be with Nana Atelier. Chan, an animator, expresses his desire to collaborate with the atelier to design clothes for his female character, Won. He wins over Na-na, and they agree to work together. However, when Chan steps out to retrieve a USB from his car, he encounters Ha-ran, causing another shock—this time, she doesn't recognize him.
They both return to the meeting, but Chan is now distracted and spills coffee on himself. Ha-ran is tasked with bringing him a new shirt and accidentally sees him shirtless, noticing a memento mori tattoo. Afterwards, Chan tells Ben he needs to leave the project and return to the US. Ben is upset by the sudden change, but Chan changes his mind as the project is only for three months.
At home, Na-na tries to set Ha-ran up with one of their photographers, but she declines. In the morning, Ha-ran's sister Ha-young, who also works at the atelier, sees a news clip about construction materials almost falling on Ha-ran. She recognizes Ha-ran, who becomes curious about Chan after seeing the clip. Both women recognize the man's shirt as the one Chan was wearing but dismiss it as a coincidence.
At work, Ha-ran finds Chan's shirt in the changing room and discovers the invitation to the painting exhibition she visited that day. Moreover, the office receives an envelope full of pictures of Ha-ran taken without her knowledge. Ha-young wonders if Ha-ran has a stalker. Ha-ran realizes that the animated character Won also looks a lot like her and wonders if the stalker could be Chan.
That evening, Chan visits the cafe where Ha-ran goes. He seems familiar with the cafe owner. They talk about how old memories resurface and how it's futile to fight them. Meanwhile, Na-na drags Ha-ran to a hotel to set her up with Photographer Yoon, but Ha-ran refuses to go. She tells her grandmother that she's scared of something bad happening to someone she cares about again. On the drive home, Na-na sees a date in her calendar marked DD and grows sad. It's just a couple of months from the present moment. She recalls how Ha-ran's parents died and how broken she was when Chan died too.
Presently, Ha-ran speaks to Photographer Yoon just as a courtesy to tell him she's not interested in dating. She then sees Chan at the hotel and gets irritated at how far he's going. She storms up to him and confronts him, asking how he knows her name. Chan is taken aback.
A flashback shows the old Chan pursuing Ha-ran in the US but not being able to find her. A voiceover claims she was the warm spring in the harshest winter of his life. We then hear Ha-ran saying "I love you" again but this time, she says the person's full name—it's not Seonu Chan but Kang Hyeok-chan.
