"UFO Ultramaiden Valkyrie 2: December Nocturne" continues the story of Kazuto Tokino, a young man who runs a bathhouse with his family. The series builds upon the events of the first season, delving deeper into the relationships and conflicts between its characters. Valkyrie, a princess from the planet Valhalla, remains a central figure, having been split into two personas: the mature Valkyrie and the childlike Valkyrie-chan. Both forms coexist due to a magical accident, creating a dynamic interplay between their personalities and their connection to Kazuto.

The narrative explores the romantic tension between Kazuto and Valkyrie, as well as the challenges posed by their intergalactic circumstances. New characters are introduced, including Hydra, a mysterious and powerful figure with ties to Valkyrie's past. Hydra's arrival brings conflict, as her intentions and abilities threaten the fragile balance of Kazuto's world. The series also introduces additional Valkyries from Valhalla, each with their own motivations and connections to the central plot.

Themes of love, identity, and sacrifice are woven throughout the story, as Kazuto and Valkyrie navigate their feelings for each other while facing external threats. The childlike Valkyrie-chan provides moments of levity and innocence, contrasting with the more serious and mature Valkyrie. The series balances comedic elements with dramatic developments, creating a blend of lighthearted and emotional moments.

As the story progresses, the stakes escalate, with battles against powerful adversaries and revelations about Valkyrie's true purpose and destiny. The relationship between Kazuto and Valkyrie is tested, as they must confront their own insecurities and the challenges posed by their differing worlds. The series concludes with a resolution that ties together the central conflicts while leaving room for further exploration of the characters' futures.