"Nine: Kanketsuhen" is a direct continuation of the original "Nine" anime, serving as its conclusive arc. The story revolves around Kakeru, a young man who discovers a mysterious device called the "Nine," which allows him to travel back in time for nine minutes. This ability becomes central to unraveling a complex web of events involving his friends, family, and a shadowy organization. The narrative delves into themes of fate, sacrifice, and the consequences of altering the past.

Kakeru's journey begins with the tragic death of his childhood friend, Haruka, which he is determined to prevent. Using the Nine, he repeatedly attempts to change the course of events, but each intervention leads to unforeseen repercussions. As he delves deeper, he uncovers connections between Haruka's death, a series of murders, and the enigmatic organization known as the "Time Research Institute." This group is revealed to be experimenting with time manipulation, and their actions have far-reaching implications for the timeline.

The plot intensifies as Kakeru teams up with his friends, including Aoi, a resourceful and determined ally, and Natsume, a mysterious girl with her own ties to the Nine. Together, they confront the Time Research Institute, uncovering its true motives and the extent of its experiments. The story explores the moral dilemmas of altering time, as Kakeru grapples with the ethical implications of his actions and the potential erasure of his own existence.

Key characters include Kakeru, whose relentless pursuit of saving Haruka drives the narrative; Haruka, whose death serves as the catalyst for the story; Aoi, who provides both emotional support and strategic insight; and Natsume, whose connection to the Nine adds layers of mystery and intrigue. The antagonist, the Time Research Institute, is portrayed as a morally ambiguous entity, with its members pursuing their goals at any cost.

The series builds toward a climactic resolution, where Kakeru must make a final, irreversible decision to either save Haruka or preserve the timeline. The ending ties together the various threads of the story, offering a poignant conclusion to the themes of time, memory, and the inevitability of loss. "Nine: Kanketsuhen" is a tightly woven narrative that combines elements of science fiction, mystery, and drama, delivering a thought-provoking exploration of time travel and its consequences.