Sanako Hase has a reputation in her school for pursuing sexual relationships with men, earning her the label of a slut from her classmates. When a group of girls accuse her of seducing boys, she runs away and meets Yuito Takashi, the student council president. Sanako is immediately smitten with Yuito and asks him out, but he rejects her, as school rules strictly prohibit relationships between boys and girls. Despite this, Sanako only wants to be Yuito's friend, and he agrees to accept her as such only if she follows one thousand rules of friendship.
Guided by Yuito's rules, Sanako learns the true meaning of friendship and hopes to win Yuito over as her friend.