Shia has been to the human world more than once, and even lived as a family. A long time ago, Shia had an accident in the underworld and was transported to the human realm. She was found in front of a rice merchant's shop, and was taken in by them. But she had no memories of anything before that moment; in other words, she had forgotten about her being a demon. She would always wake up crying and screaming "Shia" and was therefor named Shima. But with this new addition to the family, bad things started to happen. Falling from the stairs, getting threads and other accidents occured, and although nobody blamed her, Shima blamed herself for all this. She would always say bad things would happen if people would stay with her.
One day, Shima meets a young boy called Taro. While Taro is strongly against anyone from the West (his great-grandfather fought in the Aizu war), not even he can withstand Shima's charm when he first sees her and turns bright red. Taro is also the first person to scold Shima for saying she's the cause for other people's troubles and Shima is thankful for that. The two of them end up spending a lot of time together, until one day, the rice merchant's shop is destroyed and the family moves away.
Years later, Shima and Taro meet again in Tokyo; Shima is a geisha and Taro is a student in his last year before graduation. He seems to be dating another woman named Akane, as we see her and Taro having lunch, visiting plays and Akane also invits him over to meet her parents. Yet, Taro also has numerous meetings with Shima. When Taro and Akane are taking a walk together, Shima sees them and, after being insulted by Akane, runs away in tears. Taro immediately chases after her and tells her not to pay any attention to Akane's words, as she was raised in a different way as them. While Shima reveals she has been bought out, Nyaa appears and just when Taro is about to walk away, Shima asks him to meet her one last time so she can let him hear the song he taught her when they were young.
One the promised day, Shima rushes towards the meeting place, when suddenly an earthquake occurs (which Kotarou calls 'the big one'). Taro, worried about Shima, hurries back to find her under a pile of rocks. While he tries to free her, several other walls collapse on him and he passes out. Shima enrages and we see her appearance changing, turning to look like a real demon as she swipes everything away that is near them. After she comes to her senses and realizes what she has done, she plans on fleeing, only to be stopped by Taro. No matter what happens, he wants to be with her and they end up living together with their two children, Kotaroh and Shino.
All seems to go well, but as we have learned from the past, happy things do not last forever, not in Shima's case. Once again, Nyaa appears, wanting to take Shima back. He explains that in order to live, Shima needs life force and leaves with the words "You know the best source is your own kind." When Shima gets ill, the people around her blame it on exhaustion, but in fact, it is because Shima doesn't have enough life force to sustain herself. In the past, she used the life force of animals, and while she feeds on human food now, it all just isn't enough anymore. After she almost takes Kotaroh's life force, Shima decides to return with Nyaa to the underworld, because she doesn't want anybody to suffer because of her. Shima promises to come back, while Taro will wait for her at the apple tree. However, Nyaa decides to get rid of the memories, and Shima forgets everything that has happened. She doesn't respond to the name Shima anymore, says her name is Shia and has also forgotten about Taro. Yet, Taro keeps waiting for her at the apple tree..
The Aizu war was the war fought from August 23 until September 22, 1868 and part of the Boshin War.
The big one is a reference to a major earthquake that took place in Tokyo in 1923, also known as the Great Kanto Earthquake, in which more than 100,000 people were killed.