Engaged to the Unidentified Season 1

Kobeni Yonomori, who has just turned sixteen, is shocked to learn she has a fiancée named Hakuya Mitsumine who, along with his little sister Mashiro, come to live with her and her older sister Benio. As Kobeni struggles to remember just who Hakuya is, Mashiro finds herself at the mercy of the siscon tendencies of Benio. Later, both Hakuya and Mashiro transfer into Kobeni's class, with Mashiro disturbed to find Benio has a completely different personality at school.

As Mashiro tries to come to grips with Benio's popularity at school, Kobeni continues to try and remember about Hakuya. Later, Mashiro gets spooked at the thought of aliens existing. Meanwhile at school, Mashiro accidentally lets slip to the class that Hakuya is living with Kobeni.

Whilst Benio finally gets Mashiro to come to the student council room for tea, Kobeni goes with Hakuya to buy some ingredients for scones. Curious as to why Hakuya is so thoughtful of her, Kobeni asks Benio why she hasn't objected to Hakuya becoming Kobeni's fiancée. Benio explains that when Kobeni was younger, Hakuya protected her when she fell down a cliff, leading Kobeni to feel guilty that she couldn't remember it.

The morning after learning about Hakuya, Kobeni ends up with a fever due to her weak body. As Benio informs Mashiro and Hakuya about what she told Kobeni, she blames herself for Kobeni's accident back then, as she had Kobeni take her place alongside Hakuya so she could go and play. The next morning, Kobeni opts to stay at home alone whilst the others go to school, though Mashiro comes back to help her as soon as school ends, where Kobeni shows her the scar she received back then. The next morning, as Kobeni starts feeling a bit better, Hakuya suddenly rushes out of the house when he notices Kobeni struggling to face him properly. Kobeni catches up to him and states that, even though she doesn't remember anything, she owes him her thanks, becoming awed by his smile. Later that night, Mashiro and Hakuya receive a letter from their family, saying they will be sending someone to check up on them.

Kobeni learns about a member of the Mitsumine family coming over, along with the fact that Mashiro is nine years old. Whilst shopping for supplies for her guest, Kobeni comes across a young woman who had gotten lost on her way somewhere and helps her out. This woman turns out to Mashiro and Hakuya's mother, Shirayuki, who casually lets slip that their family isn't human. This causes Kobeni to have another fever as she remembers more things about her past, where Hakuya was allegedly a dog. Shirayuki later tells Kobeni that her constant fevers are the result of Hakuya giving her half of his inhuman power to save her life. Before taking her leave, Shirayuki gives Kobeni her thanks for treating Hakuya well, mentioning how he picked up his habit of stroking her head from her.

Kobeni's mother, Akane, who had always known the Mitsumines weren't human, tells Kobeni that she is no less thankful to them for saving her life. The next morning, Kobeni tries to learn more from Hakuya and Mashiro about their other forms, though Hakuya refuses to transform in front of her. As Kobeni turns her attention towards Valentine's Day, she ponders whether she should marry Hakuya or not. On Valentine's Day, Kobeni tries to work up the courage to give Hakuya some chocolate, but has trouble finding the chance. She finally gets the opportunity after her friend, Mayura Momouchi, comes over.

Student Council secretary Konoha Suetsugi feels some hostility towards Kobeni and Mashiro because of their closeness to Benio. Newspaper Club member Niko Ōno offers to help get some dirt on them, though her overenthusiasm proves too much for Konoha to keep under control. Konoha becomes more paranoid as Kobeni keeps walking in on her private outbursts. After having a brief argument with her over Benio, Mashiro suspects that Konoha is part of the same species as they are, with Konoha coming to realise the same thing.

As Kobeni decides to keep Konoha's true identity a secret from Benio, Niko continues to investigate her, despite warnings from Hakuya. Later, Mashiro and Konoha find some time alone to confirm they are from the same species, with Konoha assuming they were trying to get close to Benio. After Mashiro and Hakuya confirm this isn't the case, Konoha tells Hakuya that, as a male of the same species, he should marry her, which Kobeni overhears.

As Mashiro informs Konoha about Kobeni and Hakuya's engagement, Hakuya chases after Kobeni, assuring her that he is already engaged to her. When Niko overhears this and asks if they are dating, Hakuya denies it as he wanted to respect Kobeni's wishes about keeping the engagement a secret. Hearing about this from Niko, Konoha believes she is still in with a chance. As Kobeni asks Konoha to keep her engagement a secret, Konoha suggests that if she is against it, she should just break it off. Kobeni cries upon hearing this, and the conversation is soon interrupted by Benio. Later that night, Kobeni assures Hakuya that she isn't against their engagement, which Mashiro interprets as a love confession, encouraging Hakuya to get Kobeni something spectacular for White Day.

Benio seems to grow colder towards Hakuya, who seems a bit downhearted when Kobeni denies confessing. Meanwhile, Kobeni clears up the misunderstandings surrounding the engagement with Konoha, who attempts to stave off Niko's interest in Kobeni by giving her a plain article to write about. Later, Mashiro explains to Kobeni that Konoha's interest in marrying Hakuya was only because they are the same species, revealing that Kobeni is also similar to them due to having half of Hakuya's power. The next day, Kobeni decides to buy her and Hakuya matching handkerchieves, also buying some for Benio and Mashiro.

Hakuya is troubled over what gift he should get for Kobeni to make up for White Day. Meanwhile, Mashiro is almost spotted using her powers by Niko, who becomes determined to investigate the existence of a 'flying humanoid'. Later, Mashiro suggests that Kobeni and Hakuya go on a date, though Kobeni turns it into an outing with Mashiro. On the way home, Kobeni discovers Hakuya has a scar over his right eye, quickly coming to the conclusion that he got it from saving her. The next morning, after Hakuya and Mashiro receive another letter from their clan, Kobeni discovers they have disappeared, along with their belongings.

As Kobeni worries about Hakuya and Mashiro, the only clue she has being a text saying they had gone back to the mountains, Mayura encourages her to go to the mountains herself to learn the true story. Taking refuge from the snow, Kobeni is eventually found by Hakuya and Mashiro, who had only returned home to harvest some edible plants that only grow on their mountain. Kobeni comes down with another fever as a snowstorm picks up, with Hakuya heading off to look for help whilst Mashiro uses an opportunity to look for firewood. Delirious from her fever and worried about people leaving her, Kobeni heads off on her own in search for them, with Hakuya managing to save her from falling off a cliff. After Hakuya agrees to be more open with Kobeni, everyone returns home to their everyday lives.