"9-nine-" is a visual novel series developed by Palette, later adapted into a manga. The story is divided into multiple episodes, each focusing on a different heroine while contributing to an overarching narrative. The setting is the fictional city of Shiromitsugawa, where a mysterious artifact known as the "Souvenirs" grants individuals supernatural abilities. These artifacts are linked to a parallel world, and their misuse leads to escalating conflicts.

The protagonist, Kakeru Niimi, is an ordinary high school student who becomes involved in the supernatural events after encountering a Souvenir. His life intertwines with several heroines, each possessing unique abilities tied to the artifacts. In the first episode, "Episode 1: Sorairo Sorauta," Kakeru meets Sora Amakawa, a cheerful and energetic girl who wields the power of telekinesis. Together, they investigate the origins of the Souvenirs and confront a masked figure known as "Phantom Thief," who seeks to exploit the artifacts for personal gain.

The second episode, "Episode 2: Haruka Haruka," shifts focus to Haruka Kousaka, a reserved and studious classmate of Kakeru. Haruka's ability to manipulate time becomes central to the plot as the group uncovers deeper connections between the Souvenirs and the parallel world. The narrative explores themes of fate and the consequences of altering time.

In "Episode 3: Kokonoe Kokoro," the story centers on Kokoro Kousaka, Haruka's younger sister, who possesses the ability to heal. Her involvement reveals more about the parallel world and the true intentions of those seeking the Souvenirs. The stakes rise as the group faces increasingly dangerous adversaries.

The fourth episode, "Episode 4: Yukiiro Yukihana," introduces Yukiiro Yukihana, a mysterious transfer student with the power to manipulate memories. Her arrival brings new revelations about the Souvenirs and the parallel world, culminating in a confrontation that challenges the group's resolve.

Throughout the series, the narrative weaves together the personal struggles of the heroines with the larger mystery of the Souvenirs and the parallel world. The story balances supernatural action with character-driven drama, exploring themes of identity, responsibility, and the impact of extraordinary powers on ordinary lives. The episodic structure allows for a gradual unfolding of the overarching plot, with each installment building on the events of the previous one. The series concludes with a final episode that ties together the threads of the narrative, resolving the central conflict and providing closure for the characters.