During the Warring States period of ancient China, the land is consumed by centuries of endless warfare as seven powerful states vie for supremacy. In the kingdom of Qin, two war-orphaned slave boys, Xin and his best friend Piao, train together daily, sharing a single powerful dream: to one day rise from their lowly station and become Great Generals of the Heavens.

Their bond is shattered when a royal retainer arrives at their village, recruiting Piao to serve within the royal palace. While Xin is left behind, Piao is thrust into the dangerous heart of Qin's court politics. Not long after, a mortally wounded Piao stumbles back to Xin, barely alive. With his final breath, he entrusts Xin with a mission and a name, urging him to travel to a location deep within the mountains.

This quest leads Xin to a startling encounter with a young man who is the mirror image of his lost friend. This is Ying Zheng, the rightful but imperiled young king of Qin. Zheng shares Xin's ambition, though on an even grander scale: he dreams of setting aside the endless conflicts by conquering the six other warring states and unifying all of China under a single rule. After a violent coup d'état forces them together, Xin vows to help Zheng reclaim his throne. In exchange, Xin will be given the chance to leave his life as a slave behind and take his first, bloody step on the long road to becoming a Great General.

From this foundation, the story follows Xin's arduous journey from a reckless foot soldier to a rising commander. He must prove himself on brutal battlefields, learn the harsh strategies of war from legendary generals, and navigate the treacherous political currents of the Qin court. Central to the narrative is the complex bond between Xin, whose strength and instinct drive him forward, and the cunning and idealistic King Zheng, who faces constant threats from internal and external enemies, including powerful rival states like Zhao. Zheng is based on the real-life Ying Zheng, who would later become Qin Shi Huang, the First Emperor of China.

As the series progresses, Xin assembles his own small unit of loyal warriors, including the fierce and talented Diao, and engages in escalating military campaigns. Notable narrative arcs include the battle against the formidable Zhao general Wang Qi (Ouki) during the Initial Campaign Arc, which teaches Xin the true weight of command. This leads into the Zhao Invasion Arc, where Xin faces political treachery and brutal warfare. The story also explores the Coalition Invasion Arc, one of the largest-scale conflicts in the series, where multiple states unite to destroy Qin, testing the kingdom's survival to its breaking point. Throughout, the narrative weaves together large-scale battlefield tactics, intense duels, and the court intrigue of the State Reforms and Court Intrigue Arc, as King Zheng works to consolidate his power from within.

Across its multiple seasons, Kingdom chronicles the transformation of a nameless slave boy into a key figure in the quest to unite a fractured land, set against the grand, blood-soaked canvas of Chinese history.