In the year 2100, Dr. Amami is a scientist working in the field of alternative energy, who at the beginning of the series is on the verge of a major breakthrough in the area of "Rynax energy". His daughter Kurau accompanies him to the lab on her twelfth birthday, where an experiment goes horribly wrong and she is struck by a bolt of this energy, which disperses her into lights.

When she reforms, it is found that her body has been taken over by two Rynax, who are actually sentient life forms and not just a form of energy. However, one of the Rynax is too weak to awaken. The awakened Rynax takes on Kurau's name and identity, and though her newfound superhuman abilities make her a subject of interest for scientists, Kurau manages to live a relatively normal life. Ten years later, the second Rynax awakens and emerges as a twelve-year-old girl, whom Kurau names Christmas. The two look and interact very much like sisters.

Originally believed to be a form of energy, Rynax are actually binary life forms that exist in Pairs. These beings are made up of pure energy and are channeled from an alternate dimension, and not generated as Dr. Amami believed. This channeling proves very traumatic for the Rynax, and often results in nearly immediate death. A Rynax that has fused with a human is known as a Ryna sapien. However, fusing with a human is often dangerous, and the usual outcome is a slow, agonizing death for both human and Rynax. Kurau proves to be extremely fortunate, as the human and Rynax components of her identity live together in a symbiotic relationship. The Rynax also give her special abilities, such as amazing strength and agility, and the ability to fly, pass through solid objects and generate enough radiation to disintegrate objects instantly.

Other humans who have been merged with Rynax prove to be dangerous and destructive people, resulting in pursuit of the GPO (police force) and termination of both Rynax and human. Kurau and her Pair are pursued by the GPO under the belief that they, too, are a threat to the world. Kurau vows to protect her Pair, while at the same time trying to escape detection and live a peaceful life.