Osper hails from Mu, an undersea civilization forged when the cataclysm that destroyed their original continent forced adaptation to ocean depths. Across generations, Mu's inhabitants evolved potent psychic abilities. Victorious over his older, vengeful rival Drome in an annual tournament to determine Mu's strongest psychic, Osper triggers Drome's retaliation: the destruction of Mu's protective dome and Drome's flight to Earth. Tasked with pursuing his rival and avenging his people, Osper follows.

On Earth, Osper encounters Yumi Kaizu, daughter of the International Crusader Police (ICP) chief. Through her intervention, he joins the ICP unit tracking Drome. Osper confronts a series of escalating incidents tied to Drome's actions – resolving cases involving threats like a phantom battleship, a glowing entity in darkness, and "Poison Man #7." Each case culminates in a confrontation where Drome flees after battle. Osper wields five distinct psychic abilities in these fights, specializing primarily in teleportation.

Drome eventually activates an ultimate weapon designed to destroy Earth, leading to their final battle. Osper triumphs, compelling Drome to acknowledge defeat and confess his destructive path stemmed from his inability to surpass Osper. With his last breath, Drome reveals the weapon's location. Osper deactivates it, enabling their return to the undersea world. Before departing Earth, Osper leaves Yumi a letter expressing his hope to meet again someday as an ordinary person.

Throughout his mission, Osper maintains a strong sense of justice. His relationship with Yumi is one of significant understanding and support within the ICP framework coordinated by her father, Chief Kaizu.