Set in a not-too-distant future, Dahpne in the Brilliant Blue depicts a world where the last surviving population lives underwater in cities where waterways function as streets. The discontinuation of contact with the surface occurred several years ago during underwater experiments. The Deep Sea 9 Cities Association is the remaining hope for humanity. Ai Mayuzumi, an expert in riding water jet bikes, is the daughter of a research lab head. Following an attack by the terrorist group Error, Ai seeks the aid of the all-female counter-terrorist unit Stelnas, led by Kei. Though initially hesitant to welcome Ai on board, Stelnas members eventually acknowledge her prowess in handling a kidnapping attempt, and the exceptional trait she possesses. Additionally, the manga presents as a prequel to the anime.
