"Blue Sonnet" is a five-episode OVA series that blends supernatural elements with action and drama. The narrative centers on Sonnet L'Enfer, a young girl with extraordinary psychic abilities, who is raised by a secretive organization known as the Blue Sonnet Project. This organization trains her to become a weapon against supernatural threats. Sonnet’s life takes a dramatic turn when she is sent to Japan to eliminate a powerful psychic entity called the "Blue Sonnet," which shares her name and is believed to be a threat to humanity.

Upon arriving in Japan, Sonnet encounters Rei, a high school student who becomes entangled in her mission. Rei’s life is upended when he discovers his own latent psychic powers, which draw the attention of both Sonnet and the malevolent forces she is tasked to combat. As the story progresses, Sonnet and Rei form an uneasy alliance, navigating a world where psychic battles and hidden agendas collide. The Blue Sonnet entity, which manifests as a destructive force, becomes a focal point of the conflict, forcing Sonnet to confront her own identity and the true nature of her powers.

The series explores themes of identity, morality, and the consequences of wielding immense power. Sonnet’s internal struggle with her role as a weapon and her desire for a normal life adds depth to her character, while Rei’s journey from an ordinary teenager to a key player in the psychic war highlights the series’ focus on personal growth and responsibility. The narrative is driven by intense psychic battles, mysterious organizations, and the evolving relationship between Sonnet and Rei, culminating in a climactic confrontation that tests their resolve and challenges their understanding of the world around them.