Hisako Mochizuki works construction during the day and games online at night, and she’s content with that life until her best friend Kohaku finds a door—an object erased from the world and banned long before she was born. When Kohaku goes missing, she enters a new world to save them—literally, and metaphorically. To survive and succeed, she''''ll become a Doorkeeper: the people who make sure accidents like Kohaku''''s never happen, but she''''ll have to become much stronger and learn to use her new abilities to do it.