UFO Ultramaiden Valkyrie tells the story of Princess Valkyrie of Planet Valhalla and Kazuto Tokino, a young school boy who runs his grandfather’s bathhouse in spite of the protests from his parents and his sister Rika, who is always studying so she could get into a good high school and college, calling it a money pit. Based on an Earth where humans and aliens coexist with each other, one night, Kazuto was cleaning up the men’s bath until a spaceship fell to Earth and landed on the bathhouse and in the process mortally wounds Kazuto. The ship belonged to Valkyrie. In order to save his life, Valkyrie transfers half of her soul to Kazuto, but in the process, she turned into a smaller version of herself with the mind of an 8 year old. She can only regain her normal form by kissing Kazuto but it doesn't last long and she normally kisses him in order to protect him from danger.
As it turned out, Valkyrie learned that her father was planning a political marriage for her without her consent, feeling pressured, she fled to Earth and that’s when she crash landed on Kazuto's bathhouse. After fleeing, she soon learned that her sister Hydra followed her. Hydra followed Valkyrie to earth because she knew that if Valkyrie ran away, she would have to take her place at the wedding, which was not something she wanted to do. Because of that, she chased after her to Earth to bring her back. However, she crash landed on a temple belonging to Kazuto’s neighbor and childhood friend Akina Nanamura, who has feelings for Kazuto but never acts on these emotions, which she greatly regrets at times. Hydra tries to bring Valkyrie back by force, but that's made impossible when her powers are sealed by Akina in retaliation for crashing into her temple, making her become a child just like Valkyrie. However, unlike Valkyrie, her mind is that of her adult form, not a child's. Much like a kiss can restore Valkyrie, the seal on Hydra can be released by Akina's will. The two become roommates at the temple. Due to special circumstances, if Akina and Hydra argue too much, they can fuse into a powerful warrior called Akidra, containing the combined sum of their powers. They can still converse with each other in this state, and the fusion reverses if they become too agreeable with one another.
Over time, more visitors soon made their way to the bathhouse. First was that of Sanada, the chief maid of the royal family of Valhalla. Ms. Sanada, a cat-girl, is a faithful and very loyal servant. Upon learning of the princess’s disappearance, she tracked her down to Kazuto's house where she settled at once, considering Kazuto to be Valkyrie's official fiancé. Even seeing how Valkyrie becomes a smaller version of herself doesn't seem to trouble Ms. Sanada at all as she finds the smaller Valkyrie to be very cute. In fact, Sanada describes her as a 'Valkyrie fundamentalist'. She also fully supports her love interest in Kazuto. In order to ensure her main goal of keeping Valkyrie safe, she sometimes causes more trouble than good. She made an army of cat-girls maids out of a number of the city's girls for the crime of just being near Kazuto, but she later learned that it was a misunderstanding. When the girls are not serving as maids, they act pretty much like normal, but wearing maid outfits.
Then from out of nowhere, Laine, The youngest of the eight princesses of Valhalla, came to Earth to seek out Valkyrie due to her intense affection for her, but later develops romantic feelings toward Kazuto. She has the ability to transform her outward appearance into anyone she chooses. However, whatever appearance she takes, she will always have an antenna sticking out of her head, as well as her obvious feminine features, even when imitating a man. Pulling on the antenna will immediately change her back.
Over the course of the story, Kazuto and Valkyrie began to develops romantic feelings for each other and eventually fell in love. When Mehm, The eldest of the eight Valhallan princess, retrieved Valkyrie and Hydra, she tested Kazuto and Valkyrie’s love and decreed that Valkyrie would marry Kazuto, as she recognized Kazuto to be the only one who could truly understand and love Valkyrie. Rika is all for the marriage between Valkyrie and Kazuto, since she sees it as a way for the family to get out of debt. Of course when the marriage happens, they never really make it to the alter and have to postpone the wedding.
Over the course of the story, Kazuto and Valkyrie began to develops romantic feelings for each other and eventually fell in love. When Mehm, The eldest of the eight Valhallan princess, retrieved Valkyrie and Hydra, she tested Kazuto and Valkyrie’s love and decreed that Valkyrie would marry Kazuto, as she recognized Kazuto to be the only one who could truly understand and love Valkyrie. Rika is all for the marriage between Valkyrie and Kazuto, since she sees it as a way for the family to get out of debt. Of course when the marriage happens, they never really make it to the alter and have to postpone the wedding.
Of course as Kazuto and Valkyrie get closer, more princesses come visiting. There is Princess Chorus, who sees herself as a machine than human. Princess Inarba, who enforces the Law of Valhalla, and any offenders she meets will suffer the Punishment Dimension, which causes them to live out their greatest fears or some ironic punishment. Only Rika was able to counter Inarba's Thrust of Death, in which the impressed Inarba declares Rika to be her eventual successor, though Rika sees the training as just more homework. Princess Nesty, who is a captain of an anti-pirate space fleet and she spends her time hunting down corsairs, criminals and such. And there is Pharm, who has a fetish for women wearing large spectacles like the one she has on. She begins converting the girls at school wearing large eyeglasses, and comes into conflict with Ms. Sanada and the Cat-Girl Maid Squad. She later reveals that she wanted to see Valkyrie wear them.
Later in the story, while trying to have another attempt to get married, Valkyrie's older and younger selves split into two separate people. Valkyrie's former classmates mistake her younger self to be her daughter, until she displays knowledge about them convinces them she really is the younger Valkyrie.