Cells live inside Yumi's head, a 32 year-old who works at Daehan Noodles' finance department. In her dull life where her main concerns are what to eat or wear, only one thing excites Yumi and her cells — Woo Gi from Sales! He's handsome, gentlemanly, and 5 years younger... Age doesn't matter if he's just a crush, right? Or she thought, but Woo Gi keeps hanging around her. Is this a green light?
Yumi gets to go to the flower festival with Woo Gi, alone! However, Ruby keeps following them and disrupts the date of her dreams before it even begins. The ultimate chase between Yumi and Ruby begins. Meanwhile, Woo Gi makes up his mind to ask Yumi the question that has been on his mind.
Although his first impression was a total bust, Gu Woong turns out to be more decent than Yumi thought. Yumi's heart seemed to be opening up to the easygoing and honest Woong... But why doesn't he reply? Yumi's mind is coming to all sorts of weird conclusions to understand why Woong didn't reply!
Woong ran in the rain to bring Yumi to the hospital. Finally, the door to Yumi's heart that was shut tightly opens up again... Meanwhile, Woong's female friend Sae-yi starts developing weird feelings when Woong starts dating.
Sae-yi, the friend that Woong often mentioned, turned out to be a woman! "Can men and women be platonic friends?" The cells break into a heated discussion. On Yumi's birthday, Yumi visits Woong's game studio and finally comes face-to-face with Sae-yi at an unexpected place.
Having only dreamt of it, Woong finally visits Yumi's house. That night, Yumi and Woong spent an unimaginable night together. At Woong's suggestion, the two of them decide to go on a trip. Yumi's heart races at the thought of a hotel stay with an ocean view, even though her Travel cell is dead. Also Naughty Cell's heart is racing, too...
At a friend's wedding announcement, Yumi mentions that she has a boyfriend. When her friends don't believe her, she proclaims that she will bring Woong to the wedding. But Woong is too busy, she has no choice but to... Just as she was about to give up, another situation crops up where it becomes necessary that Woong attends the wedding.
Rumors about Yumi and Woong's wedding spread like wildfire. However, the two of them have different thoughts about marriage. Meanwhile, someone unexpected appears in front of Yumi as she prepares for a marathon. Who is this guy who found her missing umbrella...?
Yumi finally explodes in anger when Sae-yi moves houses. The breakup alarm gets activated in Yumi's cell village. They risk breaking up if Woong doesn't contact her in 24 hours. While the couple goes through a cold war, Sae-yi takes this opportunity to get between them.
Yumi's Love cell is good at everything, but she has one weakness. Thus, the cells suggest that Ruby becomes her private tutor to help Love Cell! Meanwhile, Ba-bi who has been paying attention to Yumi's social media posts, gives Yumi an unexpected suggestion, making her heart race.
As the two of them start living together suddenly, they learn about the sides of each other that they never knew existed. Both Yumi and her cells grow to like Woong even more. If Woong feels the same too, perhaps they can...
After Yumi brought up the topic of marriage, their relationship grows awkward for some reason. Meanwhile, Woong and Ba-bi attend the same dinner party by coincidence, and drunk Woong forgets everything that happened that night. What could have happened the night before? Woong feels anxious all of a sudden...
Woong breaks Yumi's precious table while changing a light bulb for her. Seeing Woong go to extreme lengths to replace her table, Yumi has mixed emotions... On the wedding day of Woong's friend, Yumi decided to go on a date with Woong after a long time.
Yumi and Woong decide to take a break. Yumi now has to spend time alone when she used to spend it with Woong, so she seeks advice from Ba-bi... After some time, the two of them meet again at the place they first met. What would Yumi's decision be? And what would Woong's decision be?
