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!

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'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.

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...

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?