The series takes place in a dystopian universe characterized by towering edifices. It centers around a particular class of beings, termed Gaunas, who possess the unique ability to mold armaments and protective gear out of bone-like material. The plot centers around one specific Gauna, Kudou Denji.
It is licensed for French release by Glénat, for Polish by JPF,[6][7] and for English by Viz Media.
Sometime in the future, humans have destroyed Earth's environment and are forced to live in cities connected only by a single highway and many tubes. However, some humans have mutated and become White Gaunas, a species which poses a threat to mankind. A group of scientists foresaw this dystopian situation years before. In an attempt to preserve humanity's future, they produced a number of time machines powered by nuclear plants called mausoleums. Each time machine can send humans to a distant future, thus ensuring the survival of the human race. They also use the collective energy from these plants to shift all White Gaunas on Earth at the time to a separate dimension. The scientists eventually entrust their knowledge to the fourth Eon Group (fourth Chronicle Group), tasked to ensure that at least a pair of humans can escape into the future.
However, White Gauna mutation is still prominent, and White Gaunas consume the remaining humans. People begin to fight over the time machines and the mausoleums in desperation. Due to the chaos, all mausoleums are destroyed except for one. The fourth Eon Group is wiped out, but not before they upload remnants of their memories and personalities onto computers embedded in the Raven, the Skull Man, and the Stick.
Moreover, the Observation Bureau (Kagen House) is created to safeguard the true purpose of the mausoleums, so as to prevent further conflict.
Hundreds of years later, the Observation Bureau study the White Gaunas to counter them and engineer a human-Gauna hybrid known as Black Gaunas, which have the abilities of White Gaunas, yet retain their human minds. Two children, Denji and Nayuta, are used as test subjects. Nayuta is the first to be experimented upon and a control device is implanted in her. However, before they can implant the other control device, Denji escapes, killing people in the process.
A few years later, Tadohomi engages Denji's help in eliminating the first White Gauna before more casualties occur. Denji complies but is wounded in the process. Sakijima tracks down Tadohomi and arrests her after Denji's body is found at the scene in the aftermath of the battle with the White Gauna. At this point, Sakijima is oblivious about the Observation Bureau's existence. Meanwhile, the Observation Bureau sends Nayuta to bring Denji back to HQ by taking advantage of Denji's wounded state. The boss of the Observation Bureau feels that Black Gaunas are untrustworthy; to him, Black Gaunas are monsters akin to their White cousins. Sakijima is ordered by his bosses in the Ministry to release Tadohomi. At this point, he starts becoming suspicious of the Observation Bureau and decides to dig up the truth.
Another White Gauna spawns, but this time it is able to consume humans before the Bureau takes action. At first, they send a squad of humans, but when this fails, they send Nayuta to destroy the big White Gauna. However, a humanoid White Gauna appears and duels with Nayuta. After a battle between the two, Nayuta wins — but falls dead soon after. Because she is telepathically linked with her twin Ayuta, any damage sustained by Nayuta is also manifested in Ayuta.
Meanwhile, the cop Sakijima gains information regarding the fourth Chronicle Group and the Gaunas from a colleague who was sent to talk with an old man who is in charge of the archives. With more information, Sakijima moves to Kegen Hall for a confrontation. His colleague is shot down by a Kegen Hall security guard whom Sakijima then kills. Sakijima meets Tadohomi who is attempting to rescue her childhood friend, Denji. Just as she is about to get killed by a team of Kegen Hall guards, Sakijima helps her out and they both enter the room where Denji is being held. Then, a revived Ayuta awakens and mutates into a White Gauna. Denji fights Ayuta while Sakijima and Tadohomi flee on a helicopter. The successors to the fourth Chronicle Group's memories seek to activate a missile weapon they believe is capable of killing the Big White Gauna if it hits the spinal cord. Ayuta, now a White Gauna, attacks them and Denji fights her off, which the Raven and the Skull use as an advantage. When the missile is primed and the Big White Gauna is in sight, the Skull is still hanging onto the missile, trying to attach the hostless Black Gauna to the projectile. Raven has no choice but to fire the missile along with him in hope of killing the Big White Gauna.
The missile hits and the Big White Gauna falls to his death. However, the nuclear reactor is beginning to fail, and the Forbidden Cage appears due to insufficient energy to dimensionally shift all White Gaunas. A White Gauna attacks the helicopter, which crashes. The Stick and the Raven guide Tadohomi and Sakijima to the House storing the time machine. The Raven is killed by Ayuta, but Nayuta appears in the nick of time and kills her sister. Tadohomi and Sakijima escape into the House just as an explosion seemingly destroys Nayuta. With the help of the Stick, the duo activate the time machine.
Many years later, the rest of humanity has been killed off either by the White Gaunas or by infighting. The world is an empty landscape, and lush greenery has taken over the House's rooms. Tadohomi and Sakijima run out of the House and look up into the sky.
Two Black Gaunas, revealed to be Denji and Nayuta, continue to battle with the White Gaunas, who are stranded in a separate dimension.
