"AIKa R-16: Virgin Mission" is a three-episode OVA series that serves as a prequel to the original "AIKa" anime. The story centers on Aika Sumeragi, a 16-year-old high school student and aspiring treasure hunter, who works part-time for the salvage company Marine Beauty. Aika is skilled in underwater exploration and combat, often using her agility and resourcefulness to navigate dangerous situations. Her mentor, Karen Minamino, oversees her training and missions, providing guidance and support.

The narrative begins with Aika and Karen tasked with retrieving a mysterious artifact from the ocean floor. The artifact, known as the "Golden Key," is believed to unlock a legendary treasure hidden in the depths. However, their mission is complicated by the appearance of rival treasure hunters and a shadowy organization with its own agenda. As Aika delves deeper into the mission, she uncovers a conspiracy involving ancient technology and a plot that could have global consequences.

Throughout the series, Aika faces numerous challenges, including underwater battles, traps, and encounters with formidable opponents. Her determination and quick thinking are put to the test as she strives to complete her mission while protecting her friends and uncovering the truth behind the artifact. The story balances action, adventure, and lighthearted moments, showcasing Aika's growth as a character and her evolving relationships with those around her.

The series also introduces supporting characters such as Erika, a fellow treasure hunter with a competitive streak, and Gonzo, a quirky inventor who provides Aika with gadgets and technical support. These characters add depth to the narrative and contribute to the dynamic interactions that drive the plot forward. The OVA maintains a focus on Aika's journey, blending high-stakes action with elements of mystery and exploration, while setting the stage for the events of the original "AIKa" series.