Momo Belia Deviluke, having fallen for Rito Yuuki herself, devises a harem plan exploiting Deviluke royal custom to let Rito marry her older sister Lala while keeping every other girl in his orbit. Meanwhile, a transfer student named Mea Kurosaki arrives at Sainan High with a hidden connection to the assassin Golden Darkness. This third anime season shifts the central heroine from Lala to Momo.