"Alien Nine" is a four-episode OVA series that blends elements of science fiction, psychological drama, and coming-of-age themes. The story is set in a world where alien creatures, known as "Borg," frequently appear on Earth, posing both threats and opportunities. To manage these encounters, schools employ students as "Alien Party" members, tasked with capturing and handling the Borg. The series primarily focuses on a group of sixth-grade girls—Yuri Otani, Kumi Kawamura, and Kasumi Tomine—who are reluctantly chosen to serve as their school's Alien Party.

Yuri, the protagonist, is a timid and fearful girl who struggles with her new responsibilities. She is deeply uncomfortable with the violent and grotesque nature of the Borg, often expressing her desire to quit. Kumi, on the other hand, approaches the role with enthusiasm and determination, displaying a strong sense of duty. Kasumi, the most reserved of the trio, often serves as a mediator, balancing Yuri's anxiety and Kumi's zeal. Each girl is paired with a symbiotic alien called a "Borg Hat," which enhances their abilities but also raises questions about autonomy and identity.

The narrative explores the psychological and emotional toll of their duties as the girls confront increasingly dangerous and surreal situations. Yuri's fear escalates as she grapples with the moral ambiguity of their tasks, particularly when faced with the possibility of harming or killing Borg. The series delves into themes of fear, responsibility, and the loss of innocence, often juxtaposing the innocence of childhood with the grim realities of their alien encounters. The Borg themselves are depicted as both menacing and pitiable, blurring the line between friend and foe.

As the story progresses, the line between human and alien begins to blur, particularly for Yuri, whose Borg Hat, named "Mr. O," starts to exert a more significant influence over her. The series culminates in a surreal and unsettling climax, leaving many questions unanswered and emphasizing the ambiguity of the characters' futures. "Alien Nine" is a thought-provoking exploration of adolescence, fear, and the unknown, presented through a lens of psychological tension and speculative fiction.