From Aku-Tenshi:
In the center of a metropolis lies a small, shaky-looking house. An abnormal high school student, Sakura Tetsu (and his family), who works day and night relentlessly to support his family's living expenses, lives there.
All is normal until one day, people from the future, alien pirates from outer space, appear. The underground kingdom claims Tetsu's land. Will he prevail in protecting his own land? Or will he surrender his land to many more invaders to come?