Hananoi-kun to Koi no Yamai, known in English as A Condition Called Love, follows Hotaru Hinase, a high school girl who has spent nearly sixteen years without romance and is content to enjoy life’s small pleasures with her beloved friends and family. She believes she is not meant to fall in love and does not even understand what the feeling is supposed to be.
Everything changes after Hotaru witnesses the painful breakup of her schoolmate Hananoi, a boy she has never spoken to before. Later, she finds him sitting alone in the snow and quietly shelters him with her umbrella. The next day, Hananoi surprises her by confessing his feelings in front of the school. Hotaru turns him down, but he refuses to give up and asks for the chance to let her know him better. From changing his hairstyle to searching for a lost hairpin in the snow, he keeps finding ways to show his sincerity.
Although Hotaru does not return his feelings at first, she decides that spending time with Hananoi might help her understand love. As they begin dating, she gradually learns what it means to be a girlfriend and how to respond to the thoughtful gestures he continues to give her.