Engaged to the Unidentified

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.