A chibi robot doll appears in the room of a typical high school boy. The robot claims the boy as her husband and then a beautiful girl comes out of the doll's mouth who knows the boy from 10 years prior and had been promised his hand in marriage.
The Yūki's residence is now full of people: in addition to Rito and Mikan Yūki, Lala, her twin sisters Nana and Momo, and Golden Darkness. Momo, who also falls for Rito, actively tries to create a harem for him. Meanwhile, a mysterious red hair girl named Mea Kurosaki enters Sainan High School with a hidden agenda.
Valkyrie, Kazuto, Akina, Hydra and friends are back for more when the Key of time suddenly appears along with many other mysterious happenings. All seemed to be linked to a person named 'Valkyrie Ghost' who's only goal is stealing Kazuto for herself. As they team up against this seeming unbeatable foe they dig deeper into the mysterious past of Valkyrie's home planet, Valhalla.
The third season doesn't have a particular story behind everything (like "Valkyrie Ghost" in the second season). It just tells 6 stories about the life of Kazuto & Co. with some additions: Raine and Chorus get a house on earth, the last three Valhalla princesses each get an episode, and in the last episode Kazuto and Valkyrie, who in the same episode managed to stay adult, try to marry again.