Cha Ji-yoon is an ordinary 30 y/o office worker who has been working at a semiconductor company for five years. Not long ago, she broke up with her boyfriend, Jo Ga-eul, an indie musician, and declared a self-imposed break from dating. Since then, she has spent a long time living that way. Believing that love makes too many things possible, she chooses not to expect anything from it anymore, enduring each day through the simple routine of going to work, eating lunch, and drinking a cup of instant coffee.

Then one day, she has a minor conflict with Kang Si-woo, the company's most notoriously cold and difficult supervisor. Ji-yoon makes a mistake, and Si-woo ends up fixing it for her. Afterwards, he unexpectedly says, “If you're sorry, you can buy me a drink.” For some reason, his remark makes her heart race. It is the beginning of a confusing feeling that she cannot tell is excitement or disbelief.

Kang Si-woo is known for his efficiency, discipline, and near-obsessive devotion to maintaining his health. On the surface, he seems blunt and heartless, but as he and Ji-yoon continue to cross paths, unexpected sides of him gradually begin to emerge. At the same time, it becomes known that he has a history of divorce with a fellow employee, a fact that helps explain why he keeps his emotions locked away so tightly.

As work repeatedly brings them together, Ji-yoon initially sees him as nothing more than an uncomfortable boss. Over time, however, she begins to find herself drawn to the person hidden beneath his cold exterior. Si-woo, too, initially treats her like any other employee, but gradually notices that he reacts differently to her than to anyone else. Both carry wounds from the past: Ji-yoon still bears the hurt left by an ex-boyfriend who feared marriage and ran away from commitment, while Si-woo struggles with the pain of his divorce. Because of those scars, neither finds it easy to acknowledge their growing feelings.

At that very moment, Jo Ga-eul reappears. In the past, he was torn between pursuing music and building a future, and his inability to take responsibility for the relationship led to their breakup. Now, however, he has achieved a certain degree of success as an indie musician. He approaches Ji-yoon again, revealing that he still has lingering feelings for her. Since she loved him deeply for a long time, his return shakes her emotionally. Yet her relationship with Si-woo is also growing deeper, and the conflict within her intensifies. The company's most difficult and cold-hearted man, Si-woo, or Ga-eul, the ex-boyfriend who once fled from the idea of marriage but has now returned - which of the two truly belongs in her future?

The story unfolds by slowly unraveling this emotional knot. As Ji-yoon spends more time with Si-woo, she begins to see the warmth and attentiveness he never shows to anyone else. Gradually, she finds herself falling for those hidden qualities. Si-woo, in turn, is drawn to Ji-yoon's honesty, her lack of pretension, and the simple, hardworking way she survives difficult days with nothing more than a cup of instant coffee. As they share their fears and old wounds, they grow closer, eventually acknowledging their feelings and becoming a couple.

When Jo Ga-eul realizes that Ji-yoon has already made her choice, he steps aside. Ji-yoon and Si-woo move forward together, overcoming the pain of their pasts and becoming irreplaceable parts of each other's lives.

The story ends happily. Ji-yoon grows from someone who feared love because of all the possibilities it carried into someone willing to embrace those possibilities once again. Today she goes to work, and tomorrow she will go to work too, but within that ordinary routine, she and Si-woo create something special together.