Lightning the Argent, this novel is set in 2181, slightly before the events that lead up to the plot described in Guilty Gear X, and is centered mostly around Ky Kiske and his job in the International Police Force (IPF).
As the novel opens, Ky begins to hear several disturbing reports - a Megadeath-class Gear in London once sealed away by Kliff Undersn seems to be awakening, a shady organization known as the Blackard Company has announced the discovery of a cure-all medicine, and tabloid reports in London also point towards a Gear masquerading as a girl terrorizing the slums of the area.
Ky heads to London to investigate the reports and rumors. A chance meeting allows him to follow Sheevus, the president of the Blackard Company, into a socialite party of the elite. There Sheevus displays the power of his company's new medicine, Vitae, rather drastically, by smashing a dog's paw with a sledgehammer and then restoring it to perfect health. Ky attempts to a put a stop to the events, but soon he is interrupted by another unwelcome guest - Sol Badguy, who has come for Sheevus's head. The ensuing fight between Ky and Sol throws the party into confusion. Abruptly, though, both Ky and Sol hear a mysterious girl's singing voice. Sol takes the chance to disappear, leaving Ky with the aftermath.
Soon afterwards, Ky goes to investigate the sources of the Gear rumor. His investigation brings him to a rundown village of squatters, who are all living where the Megadeath-class Gear is sealed. There he finds a gang of punks harassing a wheelchair-bound girl named Marina. After chasing away the punks, Ky attempts to bring Marina to a hospital, only to be stopped by more of the villagers. They redirect him to a local doctor who turns out to be Faust. One of the villagers explain that the village has a strange arrangement with the Blackard Company; in exchange for allowing the villagers to live there and providing them with necessities, the villagers agree to serve as test subjects for the Blackard Company's products. Occasionally, however, some villagers experience side effects and are carted off to the company's hospital - and never return.
After Marina recovers, she tells Ky that a strange rash of murders has been happening recently, but the bodies are always taken away by the Blackard Company before anybody can investigate. Ky decides to spend some time in the village, living in the now-abandoned church. Investigating, Ky finds that the former priest, who had raised Marina ever since she was a child, was a member of the Sacred Order of Holy Knights. He tries to investigate further, but then a loud noise awakens him.
Meanwhile Sol Badguy is wandering around, searching dead bodies, when he runs into That Man's servant, Raven. Sol confronts Raven, but Raven merely shrugs off Sol's attacks, then disappears. Ky then enters the scene and the pair fight yet again. Eventually Sol gets frustrated and runs off. Ky ends up taking one of the skeletons that Sol was searching.
The next day Ky goes to visit Marina again. It turns out that Marina has a job tutoring the kids in the village, so Ky accompanies her. That day, Ky runs into a break in his investigation when all the kids notice that every time a murder occurs, there is also a beautiful singing voice, much like the voice that Sol and Ky heard at their initial fight.
Afterwards Ky thanks Marina, and attempts to give her a bottle of Vitae that he had received from Sheevus awhile back. Marina refuses, but Ky says that with its miraculous powers, it might even be able to restore her power to walk. Marina still has doubts, but she accepts it anyway. Ky then goes to visit Faust, bringing with him the body that Sol had been investigating. Faust recognizes the body as one of his patients, who he had been treating for an injured arm. Ky notes that the bones of the arm look as if they've been chewed upon, perhaps by a Gear.
Marina has meanwhile caught a cold, and Ky drops off some cold medicine for her. Her students all are worried about her, and one of them proffers her a pill. She takes it, then realizes too late that she took the Vitae from Ky, and not the cold medicine. She visits Ky, and he's amazed at the results.
Faust meanwhile pays a surprise visit to Ky, and he points out an interesting fact - many of the murders were patients of his, and the bodies are always missing whatever part of them was diseased. Ky mentions the mysterious singing, when Faust shushes him, saying he hears it. He vanishes, Ky chasing after him. Faust has encountered Raven, and the two fight. Raven ends up victorious, however, and delivers a warning to Ky, not to let Sol get close to the source of the 'resonance'.
After patching Faust up, Ky finds some old data files left behind by the priest. As it turns out, Marina was left on the church doorstep being clutched by a dying woman, and in her hands was the sigil of Justice. The priest feared that Marina might actually be a Gear, and had been watching her. Ky, alarmed, goes off to visit Marina, but she isn't feeling so well. Suddenly she collapses, and her legs explode into writhing black tentacles. Ky hacks them off. The tentacles merge and form a woman's face, strangely similar to Marina's, and they emit the same beautiful singing that he had heard earlier before crumbling.
Ky hurriedly takes the badly wounded Marina to Faust. Then, he realizes the true power of Vitae - it is able to cure any disease or wound because it converts human cells to Gear cells. The 'murders' in the vicinity were not the result of a Gear attacking already sick humans, but rather the Gear cells in the Vitae-enhanced body running lose and destroying the host body.
The realization comes just as Sheevus has set his plan into motion. The Megadeath-class Gear fully awakens, terrorizing the village, and Ky's butler Bernard calls up reinforcements from an unlikely source - Johnny and the Jellyfish Pirates. Faust goes to hold off the Gear as well, as Ky searches for the source of the singing. He eventually runs into Sol, but this time Ky asks Sol to let him take care of it. The pair fight for a third time, and this time Sol emerges victorious, even going so far as to take the Fuuraiken from Ky. Sol eventually comes across a girl whom he recognizes to be the source of the singing, but he also recognizes her to be a Gear with free will. He's about to kill her, but something about her totally innocent nature makes him hesitate, and he spares her at the last second.
In the mean time Ky has found Sheevus and is attempting to take him into custody. However, Sheevus swallows a bunch of his own Vitae and turns into a Gear. Ky is in a pinch without his sword, when Sol suddenly appears again and tosses him the Fuuraiken. Ky manages to kill Sheevus, then turns to confront Sol. However, Sol has disappeared, leaving only the girl.
Sheevus's master plan had been to create a command-type Gear, like Justice, using a Gear with free will. The girl Sol found is named Solaria, and she is taken under the protection of the IPF. Marina recovered from her injuries and is now studying for a teaching license in London. For Ky, it's life back to normal, when he suddenly receives reports of a possible free-willed Gear in A Country, which sends him off to find her, just as told in Guilty Gear X.