"UFO Ultramaiden Valkyrie 3: Bride of Celestial Souls' Day" is the third installment in the "UFO Ultramaiden Valkyrie" series, continuing the comedic and romantic adventures of its characters. The story revolves around Kazuto Tokino, a young man who runs a bathhouse and becomes entangled in the lives of extraterrestrial beings. Valkyrie, a princess from the planet Valhalla, crash-lands on Earth and merges her soul with Kazuto's childhood friend, Akina Nanamura, due to an accident. This fusion creates a dual personality, with Akina embodying a shy, reserved demeanor and Valkyrie representing a bold, confident warrior.

In this installment, the narrative focuses on the preparations for the Celestial Souls' Day, a significant event where souls from across the universe are celebrated. Valkyrie and Kazuto face new challenges as they navigate their evolving relationship and the responsibilities tied to the festival. The story introduces additional characters, including rival suitors and new allies, who complicate the dynamics between Valkyrie, Akina, and Kazuto. Themes of love, identity, and duty are explored as the characters confront their feelings and the expectations placed upon them.

The series maintains its blend of humor, romance, and lighthearted sci-fi elements, with episodic storylines that build toward the climactic Celestial Souls' Day celebration. The interplay between Valkyrie and Akina's personalities continues to be a central focus, highlighting their struggles to coexist and their shared affection for Kazuto. The narrative also delves deeper into the lore of the Valhalla planet and its traditions, providing context for the characters' actions and motivations. Overall, the series balances its comedic tone with moments of emotional depth, offering a continuation of the characters' journeys in a universe filled with whimsy and charm.