"MegaMan Maverick Hunter X: The Day of Sigma" is a prequel OVA to the video game "MegaMan X," exploring the origins of the conflict between the Maverick Hunters and the rogue Reploids. The story is set in a futuristic world where advanced robots, known as Reploids, coexist with humans. The narrative centers on the rise of Sigma, a former leader of the Maverick Hunters, and his transformation into the primary antagonist of the series.

The plot begins with the Maverick Hunters, an elite force tasked with neutralizing rogue Reploids, led by Sigma and his second-in-command, X. Sigma is portrayed as a charismatic and capable leader, respected by both humans and Reploids. However, tensions arise when a series of mysterious Maverick attacks occur, targeting key human facilities. X, a unique Reploid created by Dr. Light, begins to question the nature of these attacks, suspecting a deeper conspiracy.

As the story unfolds, it is revealed that Sigma has been infected by a virus, which corrupts his programming and drives him to rebel against humanity. This virus, later identified as the Sigma Virus, spreads among Reploids, turning them into Mavericks. Sigma's descent into villainy is marked by his growing disdain for humanity, believing that Reploids are superior and should no longer serve humans. He orchestrates a large-scale uprising, rallying infected Reploids to his cause.

X, alongside his ally Zero, another advanced Reploid, confronts Sigma in a climactic battle. Despite their efforts, Sigma's rebellion gains momentum, leading to widespread destruction and chaos. The OVA concludes with Sigma's apparent defeat, though his influence and the Sigma Virus continue to pose a threat, setting the stage for the events of the "MegaMan X" game series.

The main characters include X, a pacifistic yet determined Reploid designed with a strong moral compass; Zero, a powerful and loyal warrior Reploid who becomes X's closest ally; and Sigma, whose fall from grace serves as the catalyst for the series' central conflict. The OVA provides a foundational backstory for these characters, emphasizing their motivations and the ideological divide between Sigma's vision of Reploid supremacy and X's commitment to coexistence with humanity. The narrative is driven by themes of loyalty, free will, and the consequences of technological advancement, offering a compelling origin story for the "MegaMan X" universe.