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?