It's the day before Kai's birthday and Kwon Ji Won has no plans and no gift for her boyfriend. Luckily he's willing to pardon her neglect on one condition: she plays his servant for twenty-four hours.
In which Hwang Jinae insistently leaves voice messages to her boyfriend every night since he's gone to Veterinary College, only to discover that she has been confessing all her struggles and hardships to the wrong number. More specifically; to his roommate Byun Baekhyun.