Sasafune Koyama is a young writer who decided to visit his grandparents for a change. When he was waiting for their return, he stumbled onto a weird wooden box filled with a manuscript titled Kakutei. It's actually the unfinished work of the ancient writer Chidori Yukimura who died from sickness at the early age of 29.
Kakutei is an inn hosted by Amanan, who lives on the life force of their guests, mainly people who get pulled into the world inside the manuscript after reading it. Unfortunately Sasafune is quite weak in body, so he won't be a very good 'food supplier.' That's why they decided to let Sasafune go as long as he brings another guest to the inn.