The story is about 16 years old boy Takeshi who recently formed an American football team at the BlueSky orphanage. One day the game is interuppted by a light in the sky. Takeshi discovers he is the son of the king of the planet Apolon, spirited away to Earth by his father's retainer Rabi to avoid death at the hands of General Dazaan. He turns out to have special energy abilities which can control UFO saucers from under the ocean floor, which in turn can become robots that are synchronized to their movements. (Source: Wikipedia)

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)

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)

UFO Princess Warukyure, aka UFO Princess Walkyrie is about a princess from outer space who accidentally crashes on earth, where Kazuto desperately tries to maintain the public bath of his grandfather. Due to circumstances, Kazuto receives part of princess Walkyrie's soul which forces her to stay there with him. But that's not the only problem ... because her soul lost strength, the princess transforms both mentally and physically into a little kid! (Source: AniDB)