Amane Fujimiya, a socially awkward first-year high school student living alone, encounters his beautiful neighbor Mahiru Shiina, who is often referred to as an "angel" by their peers. After lending her his umbrella during a rainstorm, Amane catches a cold, prompting Mahiru to care for him, leading to the development of their unique relationship.

On Valentine's Day, after the two neighbors eat dinner together as usual, Mahiru gives Amane some hot chocolate. Amane accepts it as a normal Valentine's Day gift, but Mahiru seems restless, as if she has something to tell him. When Mahiru rushes home, still unable to say what's on her mind, Amane notices a small bag with a card at his front door. Inside the bag are chocolates that Mahiru made especially for Amane, who had said that he doesn't like very sweet things. In return, Amane gives Mahiru a special present on White Day.