"AIKa: Special Trial" is a two-episode OVA series that serves as a prequel to the original "AIKa" anime. The story is set in a futuristic world where the Earth has experienced a catastrophic rise in sea levels, submerging much of the land. In this setting, Aika Sumeragi, a skilled salvage diver and adventurer, operates as a freelancer alongside her assistant, Karen Minamino. The narrative begins with Aika and Karen being hired by a mysterious client to retrieve a valuable artifact from the sunken ruins of Tokyo. The mission quickly escalates into a high-stakes adventure as they encounter rival treasure hunters, dangerous underwater traps, and a secretive organization with its own agenda.

The plot thickens when Aika discovers that the artifact they are seeking is linked to a powerful and ancient technology capable of altering the world's balance of power. As Aika and Karen delve deeper into the ruins, they uncover a conspiracy involving a shadowy group attempting to harness this technology for their own purposes. The series balances action, suspense, and humor, with Aika's resourcefulness and combat skills being put to the test as she navigates both physical challenges and moral dilemmas.

Key characters include Aika Sumeragi, the protagonist known for her bravery, intelligence, and occasional recklessness; Karen Minamino, her loyal and tech-savvy assistant who provides crucial support; and Erika, a rival treasure hunter whose motivations remain ambiguous. The interactions between these characters drive the narrative forward, with their contrasting personalities adding depth to the story. The OVA maintains a brisk pace, blending underwater exploration, high-octane action sequences, and moments of levity, all while expanding the lore of the "AIKa" universe. The series concludes with a setup for future adventures, leaving room for further exploration of its world and characters.