Ryūka Tatsuhara always dreamt of leaving her boring hometown and starting a new life in Tokyo. But then strange doors called Daimen appeared, bringing kaijus onto Earth.

One of them, a massive kaiju known as the Daikaiju, arrived in Ryūka’s town looking for a host to carry his child. When he couldn’t find one, he decided he would flood the world with magma before moving on to another planet. But before he could follow through, his child wandered off, found Ryūka, and imprinted on her.

When he sees that his child has grown fond of Ryūka, the Daikaiju decides to hold off on destroying the world - on one condition. Ryūka has to take care of the little troublemaker until his armor plating fully develops, which will take exactly one year. After that, the Daikaiju will finish what he started, covering the Earth in magma so his child can grow up in the perfect environment.

And just like that, Ryūka is stuck taking care of a baby kaiju for a year.