Sekainohate de Aimashou

Yona Ryouma, a normal high school boy, saves a man from another world at the cost of his own life when a mysterious earthquake occurs. The man, Emillio, uses a potion which apparently saves Ryouma's life. However, he is also changed into a girl in the process.

Emillio is actually the prince of this alternate world and he declares Ryouma as his wife, presumably on the context of saving his life. But what about Ryouma's life as a high school boy in his own world? And will he come to accept being the prince's wife?