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" whose 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. (Source: ANN)

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. (Source: ANN)

The strange and magical events of the last season have left Kazuto with two Valkyries. He now finds himself living with and looking after both a teenaged-princess Valkyrie, as well as the child-princess Valkyrie! And they still have not yet gotten married... (Source: AniDB)

Fantasies and miracles never interested Ayu Tateishi, a popular second-year student at her middle school. She was content with preserving her image of being cool, calm, and collected—all to catch the eye of her crush, Tetsushi Kaji. Ayu's carefree youthful days are interrupted by the appearance of Nina Sakura, a new transfer student who turns out to be a witch. As the only person who knows about Nina's ability to cast spells using a mini-computer, Ayu is forced to help Nina with her mission to find the five Holy Stones, qualifying her to marry the prince of the Magic Kingdom. Unfortunately, Nina's ineptitude with magic and her habit of meddling in Ayu's personal life cause nothing but trouble for the two of them as Ayu toils to maintain her good reputation and Nina struggles to hide her secret from the discerning eye of Hiroki Tsujiai, Tetsushi's best friend and an avid fan of manga. And when Nina's search for the five Holy Stones brings allies and challengers from the Magic Kingdom, Ayu is dragged into a world she never even dreamed was possible. [Written by MAL Rewrite]