Invaders of the Rokujouma

After Koutarou Satomi's father is suddenly relocated for his job, the first-year high school student is faced with finding a cheap place to live by himself. Naturally, he jumps at the chance to move into Corona House's Room 106 for a mere five thousand yen a month. But while everything goes well at first, Koutarou soon gets a lot more than he bargained for after stumbling upon a mysterious cave while working his part-time job.