Amane Fujimiya is a socially awkward first-year high school student who lives by himself in an apartment. His neighbor is schoolmate Mahiru Shiina, a girl who is so beautiful and so unreachable that the other students call her an "angel." On the way home one day, Amane notices Mahiru sitting alone on a park swing, drenched in the rain, so he lends her his umbrella. When Amane catches a cold the next day, Mahiru insists on taking care of him, and the two next-door strangers embark on an unusual new relationship.

Amane's mother, Shihoko, arrives at the apartment without warning while Mahiru is visiting. Amane asks Mahiru to hide, but Shihoko eventually finds her, and the two neighbors frantically try to correct Shihoko's assumption that they are dating. Nonetheless, Mahiru is touched by Shihoko's motherly kindness. After Amane sees how happy Mahiru is to be called by her first name, he tries calling her "Mahiru" too. From that day on, the neighbors call each other by their first names.