Controlled by a complex network of cells, every human being on the planet is kept alive by an invisible operating system that is, for the most part, completely overlooked. Although humans do not pay much attention to how they manage to function on a day-to-day basis, the cells in the human body are constantly at work, doing their bit to keep their host alive and healthy. This is why, on the surface, Kim Yy Mi (Kim Go Eun) appears to be just like any other working woman in the world, but beneath this ordinary exterior lies something truly extraordinary. Having recently suffered a painful break-up, Yu Mi’s love cells have fallen into a deep coma. Whilst the cells around her work hard to revive their fallen companions, Yu Mi spends her days trapped in the daily grind. With her heart still broken and no prospect of romantic encounters, Yu Mi’s love cells might well have been in serious danger had it not been for the sudden appearance of Goo Woong (Ahn Bo Hyun). A games developer with emotional issues, Goo Woong has no idea how to express his true feelings, but after meeting Yu Mi, he’ll discover he wants to try. As his love cells unexpectedly bring Yu Mi’s back to life, things between the two begin to get interesting. Yu Mi and Goo Woong’s cells are working overtime, and the pair find themselves growing even closer. But will the efforts of this small number of cells really be enough to unite two hearts forever?