The story of an ordinary office worker named Yumi-from the point of view of the many brain cells in her head controlling her every thought, feeling, and action.
Cast:Kim Go-eun , Park Jin-young , Ahn Bo-Hyun , Lee Yoo-Bi , Jung Soon-won , Sang-yi Lee , Joo Jong-hyuk , Shin Ye-eun , Mi-Ram , Ji-Hyun Park , Pyo Ji-hoon , Jun Suk-ho , Jong-Hyuk Lee , Ki-Hwan Shin , Choi Minho
Yu Mi is having a hard time getting over Goo Woong, both physically and mentally. While dealing with her broken heart, she finds herself thinking about her co-worker, handsome and considerate Yu Ba Bi. Meanwhile, Yu Mi's cells are working intensely to cure the after-effects of her breakup.
Yu Mi rejects Ba Bi's love confession while her love cell remains in coma. However, she feels confused by her feelings toward Ba Bi after she sees him with another woman. Meanwhile, Yu Mi's cells keep fighting whether Ba Bi is the right man for her or not.
Yu-Mi feels annoyed by Ruby sending Ba-Bi on a blind date with her friend. After finding out the truth about Ba-Bi's blind date, Yu-Mi confesses her feelings toward him, and the two start dating. Meanwhile, Yu-Mi's love cell keeps questioning if Babi is really a gentleman, calm and upright person while other cells are celebrating Yu-Mi's new relationship.
Yu Mi spends the night at Ba Bi's place after experiencing a power outage at her apartment. Slowly, they are getting to know each other as they enjoy their time together. Later on, Yu Mi turns in her resignation letter to pursue her dream and celebrates her first Christmas with Ba Bi. Meanwhile, Yu Mi's love cell visits Ba Bi's village.
Yu Mi feels uneasy and is unable to stop thinking about Goo Woong after their unexpected meeting. As she spends her first Christmas at Ba Bi's place, she's disturbed by a text from Woong. Meanwhile, Goo Woong remembers his last few weeks with Yu Mi before their break up.
Yu Mi starts to feel anxious about her future as a writer after meeting her former colleagues who are all getting promoted and doing better than ever. Meanwhile, Yu Mi's parents finally meet Ba Bi but their dinner together doesn't go as planned.
Yu Mi begins working part-time after her unemployment starts affecting her savings while Ba Bi got promoted to Team Leader at Jeju's Branch. Later on, Yu Mi finally experiences her own turning point after her story leaves a deep impression on Julie Literature's editor-in-chief. Meanwhile, Yu Mi becomes bothered by Ba Bi's young intern Da Eun.
Yu Mi can't stop comparing the start of her romance with Ba Bi to his current relationship with Da Eun. After getting hired as a writer for Julie Literature, Yu Mi is taken back by editor-in-chief Ahn Dae Yong's cold and strict behavior toward her. Meanwhile, Yu Mi's relationship with Ba Bi starts to fall apart.
Yu Mi is left irritated and annoyed after meeting Control Z, the new eccentric illustrator of her debut novel. While spending her second Christmas with Ba Bi on Jeju Island, Yu Mi is faced with a painful truth.
Yu Mi is living a completely different life after her breakup with Ba Bi. Her debut book is selling well, and her relationship with Control Z is slowly getting better. Meanwhile, Yu Mi meets Goo Woong who is now a successful CEO.
In order to avoid running into Ba Bi who returned back to Seoul, Yu Mi makes a list of places he often goes to. However, her plan turns out unsuccessful. Ba Bi confesses that he cannot get over Yu Mi which results in them getting back together. Goo Woong meets Ba Bi.
Goo Woong and Ba Bi quarrel until Yu Mi steps in. Later on, Ba Bi has to leave for a three-week business trip to the United States. However, he is seen by Yu Mi in Seoul days later.
News about Ba Bi's proposal to Yu Mi spread fast. Ba Bi takes Yu Mi to his family's restaurant for the very first time as they prepare for a new future together. Meanwhile, Goo Woong continues to invade Yu Mi's thoughts and confuse her feelings.
A lost laptop reunites Yu Mi and Ba Bi months after their break-up. After a small chat, Ba Bi leaves for Singapore as Yu Mi returns home and finds out that Goo Woong bought her a new laptop. While trying to return it, Yu Mi finds out that Goo Woong never missed a single upload. After attending Ruby's party, her Love Cell returns as she receives a text from an unknown contact asking if she got home safely.