"Steel Angel Kurumi Zero" is a four-episode OVA series that serves as a prequel to the original "Steel Angel Kurumi" anime. Set in an alternate Taisho-era Japan, the story revolves around the creation and early adventures of Kurumi, a highly advanced combat android known as a Steel Angel. The narrative begins with Dr. Ayanokouji, a brilliant scientist, who develops Kurumi as part of a secret military project. Kurumi is designed to be a weapon of unparalleled power, but her creation is complicated by the integration of a mysterious energy source called the "Angel Heart," which grants her sentience and emotions.

The plot unfolds as Kurumi is activated prematurely during an attack on Dr. Ayanokouji's laboratory by a rival faction seeking to steal the Angel Heart technology. Despite her incomplete programming, Kurumi demonstrates immense combat prowess, protecting the doctor and escaping the facility. However, her lack of full emotional and cognitive development leads to unpredictable behavior, as she struggles to understand her purpose and the world around her. Throughout the series, Kurumi forms a bond with Dr. Ayanokouji, who becomes a father figure to her, guiding her as she navigates her newfound humanity.

The story also introduces other key characters, including Saki, a rogue Steel Angel who opposes Kurumi, and Nakahito, a young boy who becomes entangled in Kurumi's journey. Saki serves as both an antagonist and a foil to Kurumi, representing the darker potential of the Steel Angel technology. Her motivations are driven by a desire for freedom and autonomy, contrasting with Kurumi's initial innocence and reliance on Dr. Ayanokouji. Nakahito, on the other hand, provides a human perspective, highlighting the ethical dilemmas surrounding the creation and use of sentient androids.

As the series progresses, Kurumi's growth is central to the narrative. Her interactions with humans and other Steel Angels challenge her understanding of loyalty, morality, and identity. The OVA explores themes of artificial intelligence, the nature of humanity, and the consequences of technological advancement, all set against the backdrop of a historical-fantasy world. The story culminates in a climactic battle that tests Kurumi's resolve and her bond with Dr. Ayanokouji, ultimately shaping her into the character seen in the original series. The OVA provides essential backstory and context for Kurumi's origins, enriching the broader narrative of the "Steel Angel Kurumi" franchise.