In a near-future world where extended reality technology has evolved beyond augmented and virtual reality, the sport of gladiatorial combat has been reborn as King of Gladiator, or KOG. This technology perfectly replicates the weight, sharpness, and feel of real weapons in a virtual arena, making KOG the planet's most popular entertainment spectacle. Two years prior to the story, the sport's greatest superstar, Himemiya Ace, was struck by a truck and fell into a coma. Her identical twin sister, Jiita, a vulgar and obnoxious online streamer known as Bad_Flower5963, has since become a hikikomori recluse, living off her sister’s savings.

The story begins when Jiita is approached by Lucy, a secretary for CGG Technica, the company that operates KOG. Lucy, who harbored unrequited romantic feelings for Ace, reveals that cutting-edge regenerative treatment could awaken Ace, but it requires a compatible cell donor. To prepare her body to be that donor, Jiita must step into the arena and compete as her sister. Reluctantly, Jiita agrees, donning a special contact lens that displays video game-like menus, stats, and a life point counter as she prepares to fight under Ace's name.

The central conflict arises from Jiita’s dual struggle: she must win matches to raise her cellular compatibility rate from 99% to 100% in order to save her sister, all while hiding her true identity from the world and overcoming her own withdrawn nature as a former shut-in. Her fighting style reflects her past as a trolling streamer; she excels not only at physical combat but also at using provocative taunts to unnerve her opponents and manipulate the audience's online comments to her advantage.

The main cast includes Himemiya Ace, the undefeated legendary gladiator whose sheer strength made her the starting point or the A of the sisters’ dynamic. Her sister, Himemiya Jiita, is the Z who must finish what Ace started. Seemingly weaker initially, Jiita possesses raw talent and superhuman reflexes that even surpass her sister’s, as suggested by Lucy. Their relationship is close, with memories of their shared training and childhood bonding driving Jiita forward. Lucy acts as Jiita’s strategist and love interest, managing the secret of the replacement while dealing with her own complex feelings for Ace.

The series is set within the massive digital arenas of the KOG, which are filled with spectators wearing VR headsets and floating comment screens, as well as the Gladiator Training School where Jiita lives and trains. Her first major opponent is Rekka Hilda, a violent master-class gladiator wielding a giant axe, whom Jiita defeats with a signature move called Ace Breaker. Following this, she faces the ladylike and fancifully dressed Valeria, who wishes to form an idol group; a match where Jiita wins by rattling her opponent with crude insults rather than pure strength. After these victories, Jiita moves up the ranks to face the Gladiator Five, the top warriors in the league. She first battles Scarlet, a genius who freely manipulates weapons and uses outdated internet slang, resulting in a draw. Finally, she must confront the number one ranked gladiator, Ren, known as the Sword Saint of God Speed. Ren is the only fighter who ever managed to draw against Ace, and she shares Jiita’s tactical style of aiming for instant critical hits, making her the final obstacle to reaching 100% compatibility and awakening Ace.