In Horimiya, Kyoko Hori is a bright and popular high school student who is well-liked by her peers. However, she keeps a secret about her home life where she dresses down and takes care of her younger brother, Souta. One day, her classmate Izumi Miyamura, who is known for his nerdy appearance, comes over to her house with her brother and recognizes her despite her casual outfit. Miyamura's appearance also contrasts his usual look, as he is seen with numerous piercings and tattoos. Together, they agree to keep their after-school identities a secret. As they get to know each other better, they discover they have more in common than they thought, eventually become closer.