Gong Woo-jin (Yang Se-jong) is a 30-year-old single man who works as a set designer. Due to a trauma he experienced 13 years ago, he does not want to have a relationship with others. When Woo Seo-ri (Shin Hye-sun) was 17, she fell into a coma. Thirteen years later, she wakes up from her coma, but she is now 30-years-old. Gong Woo-jin and Woo Seo-ri get involved with each other and fall in love.
Gong Woo-jin caused an accident when he was 17 years old, or so he believes. Due to the crush he had on a girl of his age, he caused her to stay on the bus she was travelling for one stop longer than required. As a result, she was still on the bus when it crashed, and later Gong Woo-jin finds out that this girl (No Soo-mi) died in the accident. He feels very guilty and goes away to study in Germany, and carries this guilt with him into adulthood. As a result, he becomes a very strange adult. His habits include getting out a tape to make measurements for his set design miniatures at any given moment. He often gets into trouble for this habit, and he's also cold and rude to others. He even leaves Korea for long periods of time to go on trips without a word to anyone.
Woo Seo-ri wakes up in hospital confused to find that she has aged 13 years overnight. She last remembers being 17 years old, wearing her friend No Soo-mi's jacket since she forgot to pick up her own. She was in a coma, and woke up after 13 years to find herself much older and very weak. Sadly, she has no visitors, and the staff can't even find out where her relatives (her Aunt and Uncle; her parents died when she was young) are. Seo-ri finds this out after overhearing the nurse who keeps fobbing her off with excuses talking about how they say this to patients whose relatives don't come by anymore.
She decides to take matters into her own hands and search for her Aunt and Uncle. She goes to the last place she remembers them - their home - but the city is hugely changed from how she remembers. By a twist of fate, her Aunt and Uncle no longer live there: they sold the house to Woo-jin's father, who is now letting his adult son stay there while he's back in Korea. Woo-jin is also looking after his nephew, Yoo Chan (Ahn Hyo-seop), an energetic and friendly guy who instantly wants to take care of Seo-ri, despite Woo-jin's misgivings. Neither of them recognize one another at first, but Woo-jin agrees to let her stay for just one night so that she can begin the search for her relatives.
Meanwhile, at the hospital, a doctor is dismayed to hear that Seo-ri has gone missing and begins to search for her. The next day, he asks about her at her old home - but a misunderstanding doesn't let them meet.
The plot follows Seo-ri's search for her relatives; the budding romances between Seo-ri and Woo-jin, but also Yoo Chan and the doctor, who both develop ill-fated feelings for her; Woo-jin's transformation from a surly and closed-off set designer to someone who dares to love again; Yoo Chan's rowing championships; and their strange housekeeper Jennifer (Ye Ji-won), who ends up being tied up in the accident thanks to her past as well.
An aspiring violinist whose dreams were destroyed due to an accident. She got into a bus accident when she was 17years old and woke up from a coma after 13 years. She still feels she is 17 even though she now is 30 years old. She has been looking for her uncle and aunt who did not come to visit her and even sold their house where Seo-ri lived with them. She later develops feelings for Woo-jin unaware of their past.
A stage designer who maintains a detached and aloof behavior. He goes to great lengths to keep himself from getting involved with those around him such as by wearing earphones, blocking the clients from contacting him and by putting on an obtuse personality and generally just ignoring social cues. Because of his occupation, he has developed a habit of measuring random items in public with a measuring tape.
When he was younger, he had a crush on a girl who he learned was named No Soo-mi because of the uniform name tag not knowing that it was borrowed. On the day of their first interaction, she was involved in an accident which resulted in several deaths. Woo-jin blames himself for what happened to her and so he decided not to become involved with others in any way in the future. He later develops feelings for Seo-ri unaware of who in the past she actually was for him.
A well-versed lady, skilled in martial arts and always focused on the work she does, she has modern charming looks although she wears the same attire and is in her 40's. She has a strange personality. She is also the housekeeper of the house where Gong Woo-jin and his nephew reside. She harbors a secret that indirectly ties her with the accident 13 years ago.
Seo-ri's best friend. She died in the accident. She lent her school uniform to Seo-ri causing Woo-jin to mistake Seo-ri's name for her best friend's name.
^ abIn order to circumvent Korean laws that prevent commercial breaks in the middle of an episode, what would previously have been aired as single 70 minute episodes are now being repackaged as two 35 minute episodes, with two episodes being shown each night with a commercial break between the two.[2]
^ abNo episodes were aired on 20 and 27 August due to coverage of the 2018 Asian Games.
^Originally scheduled to be pushed back to 10.30pm, episodes 17 and 18 were later reverted to air at the original time instead.
^Jeong, Hee-yeon (January 1, 2019). [제56회 대종상]'기생충' 작품상 포함 5관왕 달성…이병헌·정유미 주연상(종합) [[Comprehensive]'SBS Drama Awards' Gam Woo-sung-Kim Sun-ah, Best Couple Award followed by'Joint Grand Prix']. Sports Donga (in Korean). Archived from the original on June 11, 2020. Retrieved June 11, 2020 – via Naver.
