"Alien Nine" is a science fiction manga series set in a world where extraterrestrial beings, referred to as "aliens," frequently appear on Earth. The story centers on Yuri Otani, a timid and reluctant sixth-grade student who is chosen to serve as an "Alien Party" member at her school. The Alien Party is a group of students tasked with capturing and managing aliens that appear on school grounds, using specialized equipment and symbiotic alien partners called "Borgs." Yuri is paired with a Borg named Kumi, a small, frog-like creature that attaches to her head and provides her with enhanced abilities, though Yuri finds the arrangement unsettling.

As Yuri and her fellow Alien Party members, Kasumi Tomine and Megumi Hisakawa, navigate their duties, they encounter increasingly bizarre and dangerous alien encounters. The aliens range from harmless creatures to hostile entities that pose significant threats. The girls must confront their fears and insecurities while dealing with the physical and emotional toll of their responsibilities. Yuri, in particular, struggles with her fear of aliens and her reluctance to embrace her role, often feeling overwhelmed by the violence and strangeness of her experiences.

The narrative explores themes of childhood innocence, fear, and the loss of innocence as the characters are forced to confront the darker aspects of their world. The story delves into the psychological impact of their duties, with Yuri's internal conflict serving as a central focus. Her relationship with Kumi evolves over time, reflecting her growing acceptance of her role, though not without significant emotional strain.

The series also introduces a mysterious alien known as the "Alien Queen," a powerful and enigmatic entity that plays a pivotal role in the story. The Alien Queen's presence raises questions about the true nature of the aliens and their connection to humanity. As the plot progresses, the boundaries between humans and aliens blur, leading to unsettling revelations about the characters and their world.

"Alien Nine" combines elements of science fiction, horror, and psychological drama, creating a tense and thought-provoking narrative. The series is notable for its exploration of the emotional and psychological challenges faced by its young protagonists, as well as its imaginative and often unsettling depiction of alien encounters. The story's focus on character development and its exploration of complex themes make it a distinctive entry in the science fiction genre.