"Sample H" is a manga series that centers on the life of Haruto, a high school student who discovers a mysterious device capable of altering reality. The device, known as the "Sample H," is a prototype created by a secretive organization experimenting with dimensional manipulation. Haruto stumbles upon it during a routine school trip to a research facility, and its activation triggers a series of unforeseen events. The device grants him the ability to rewrite small aspects of reality, but each use comes with a cost—gradual deterioration of his own memories and sense of self.

As Haruto begins to experiment with the device, he attracts the attention of the organization behind its creation. They send agents to retrieve it, leading to a tense cat-and-mouse game. Among these agents is Aiko, a skilled operative who initially views Haruto as a threat but gradually becomes conflicted as she learns more about his motivations. Haruto uses the device to help friends and strangers, attempting to fix personal tragedies and injustices, but his actions inadvertently destabilize the fabric of reality, causing rifts between dimensions.

The narrative explores the ethical dilemmas of wielding such power, as Haruto struggles to balance his desire to help others with the unintended consequences of his actions. His relationships with classmates, particularly his childhood friend Rina, become strained as his memories fade and his identity begins to blur. The story delves into themes of sacrifice, responsibility, and the fragility of human connections, as Haruto faces the ultimate choice: continue using the device to alter reality or destroy it to prevent further chaos.

The manga features a mix of science fiction, psychological drama, and action, with a focus on character development and moral complexity. The pacing alternates between intense confrontations with the organization and quieter, introspective moments that highlight Haruto's internal conflict. The series builds toward a climactic finale where Haruto must confront the true nature of the Sample H and its creators, leading to a resolution that challenges the boundaries of free will and destiny.