There is nothing in this world. Just endlessly falling rain. The population here is exactly one. Getting off the elevator one day, a mysterious world was waiting. No one else was around, only endless rainy sky and weathered flagstones. In this world, getting hungry and even death wasn't possible. It was a world where time seems to have stopped. Except one thing -- the rain continued to fall. "This world has a population of just one. So get out." A girl named "Rin" told the protagonist. She declared that she was the lone mistress of this rainy world. For how many years, decades, even centuries... Has she lived all alone in this world? Will the day come when the rain stops falling?