Rito Yuuki never seems to catch a break—continually he's getting into sexual mishaps with the females around him. Although his heart still yearns for Haruna, his childhood sweetheart, Rito can't help but wonder about his feelings for Lala, the extraterrestrial princess who materialized in front of him and stated her intention to marry him. But it's not only Lala he has to deal with now: her younger twin sisters, Momo and Nana, have also gone to Earth to meet their elder sister's fiancé, and they wind up living at Rito's house by chance. Meanwhile, Yami, the human weapon girl, continues her search for Rito among the bustle of his new family members. Rito's life is not simple since he has to deal with uncertainty.