Go Dok-mi (in Korean, literally "lonely beauty") is a shy, frugal freelance copy editor who, for a yet-to-be-revealed reason, refuses to leave her apartment or interact with people as much as possible. Every day, she anonymously receives a carton of milk with a post-it attached. Every day, using a pair of yellow binoculars, she steals peeks at her neighbor across the street, Han Tae-joon, as he goes through his daily morning routine. The last time she was outside, in a park on an autumn day, she had fallen in love with Tae-joon at first sight, following him and watching as he picked up a puppy in a box and took it home. When she looked out her window and saw him living in the apartment opposite hers, she thought of it as fate.
Webtoon artist Oh Jin-rak lives in the same apartment building as Dok-mi. He and his drawing partner Oh Dong-hoon have just finished their latest webtoon "Zombie Soccer," but it wasn't picked up because it supposedly plagiarized a video game by Enrique Geum. Jin-rak pitches a new story to persuade his editor Kim Seul-gi: the daily life of this one girl trapped in her apartment, and how her next-door neighbor draws her out into the world... and it'll be called "Flower Boy Next Door."
Enrique Geum arrives in Seoul from Spain, his reason for coming is "cupid's arrow." Though he's in love with his best friend Yoon Seo-young, she has feelings instead for his older brother, Han Tae-joon, so he intends to play cupid for the two.
When Enrique comes to stay at Tae-joon's apartment, he catches Dok-mi in the act. That same day, a new neighbor Watanabe Ryu moves into the apartment across to Dok-mi's (Jin-rak lives next door). And with all these new men suddenly entering her life, Dok-mi's solitary and orderly world is turned upside down.