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.

When Itsuki and Chitose visit Amane's apartment for a Christmas party only to discover that the angel Mahiru lives next door, they instantly become very curious about the relationship. Chitose, obsessed with cuteness, insists on becoming friends with Mahiru. The day after Christmas, Amane notices that Mahiru has a fever. Despite Mahiru's stubbornness, Amane convinces her to stay and rest in his room. When Mahiru sees Amane so diligently taking care of her as if returning the favor she did for him that one memorable day, it seems to put her at ease.

Mahiru and Amane ring in the new year together. When the exhaustion from cooking catches up with Mahiru and she falls asleep in Amane's arms, Amane has no choice but to let her stay the night in his bed. The sight of her sleeping peacefully but totally unguarded gives him a bit of a jolt. Two days later, Amane's parents come to visit. When the four go to Hatsumode (the first shrine visit of the year), Amane is taken aback by how pretty Mahiru looks in a kimono. Meanwhile, Mahiru keeps blushing at the sight of Amane dressed up so handsomely.

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.

The day before spring break, Itsuki has a fight with his father and asks Amane to let him stay over. When Chitose finds out that her boyfriend will get to enjoy Mahiru's cooking, she demands to join them and stay at Mahiru's apartment. The four friends spend the next few lively days together. After Itsuki and Chitose leave, Amane returns home one day to find Mahiru confronted by her mother, Sayo. Noticing Mahiru's dark expression and sensing that something is wrong, Amane invites Mahiru in. The two sit together on Amane's sofa as Mahiru timidly explains her complicated history with her parents.