"Busou Shinki: Moon Angel" is a three-episode OVA series that expands on the "Busou Shinki" franchise, which revolves around miniature humanoid robots known as Shinki. These Shinki are designed for both companionship and combat, often engaging in battles to prove their strength and loyalty to their masters. The story centers on four Shinki—Ach, Arnval, Strarf, and Altlene—who live with their owner, a young man named Kousaka.

The narrative begins with the Shinki preparing for a special event called the Moon Angel Festival, a competition where Shinki battle to earn the title of Moon Angel. Each Shinki has a distinct personality and fighting style: Ach is cheerful and energetic, Arnval is calm and strategic, Strarf is serious and disciplined, and Altlene is gentle yet determined. As the festival approaches, the Shinki train rigorously, showcasing their unique abilities and deepening their bonds with Kousaka.

The competition itself serves as the central focus of the series, with the Shinki facing off against formidable opponents in intense battles. The matches highlight their combat skills, teamwork, and individual growth. Throughout the tournament, the Shinki confront challenges that test their resolve, including rivalries, self-doubt, and the pressure to succeed. The series also explores the dynamics between the Shinki and their owner, emphasizing themes of trust, loyalty, and mutual support.

The OVA maintains a balance between action and character-driven moments, providing insight into the Shinki's personalities and their relationships with one another. While the story is relatively straightforward, it effectively captures the essence of the "Busou Shinki" universe, focusing on the Shinki's dedication to their master and their desire to prove themselves in battle. The series concludes with the resolution of the Moon Angel Festival, leaving the Shinki's future adventures open-ended.