"Jae-hoon" (Kim Rae-won) is reluctant to part with his ex-girlfriend. In the morning, which starts with a hangover as usual, he finds that he has a 2-hour call record with someone with a phone number he doesn't recognize. He finds out that the person on the other end of the phone is "Sun-young", a colleague who has only known each other's names for less than 24 hours. "Sun-young" (Kong Hyo-jin) is in the middle of a breakup with her boyfriend. On the first day of work at the new company, she said everything she should and shouldn't say when she broke up with her boyfriend. But it was seen by "Jae-hoon" who works in the same workplace. They have only met for one day, but they know each other's love history better than work. But the subtle tension and awkwardness are temporary. Even the "unreasonable" and "speechless" collisions are still concerned. Why?