The house of ordinary Japanese schoolboy Rito Yuki has long turned into a forward outpost of an interstellar empire, and the owner himself is in deep contemplation. How does one choose between childhood friend Haruna and Princess Lala when almost half of the surrounding female population is simultaneously pursuing you? Young Princess Momo, a born schemer who has come to help her older sister, suggests that the hapless Yuki openly declare himself a galactic prince, to whom a harem is by definition assigned. Then all the interested ladies will quickly sort themselves out, and Rito will gain a long, though unlikely to be peaceful, life! Exactly, for the chosen ones will inevitably include the lovely blonde Yami, hired by Lala"s rejected suitors to "deal with" some insignificant Earthling. For obvious reasons, the famous assassin is not in a hurry to carry out the order, although she once tried to feed poor Rito lunch. However, the syndicate "Darkness" that sent her is not satisfied with a loss of face, and to rectify the situation its mistress intends to take decisive measures, up to personal intervention. Naive woman! After all, everyone knows that a real harem leader is not something you can take with a gun – you can"t get one by a Big Bang!