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...