"Jaku-San-Sei Million Arthur" is a fantasy anime series based on the mobile game "Million Arthur" by Square Enix. The story revolves around the legend of Excalibur, a sacred sword that chooses its wielder to become the true King Arthur. However, due to a mysterious anomaly, multiple Excaliburs appear, each selecting a different individual as a potential king. These chosen individuals, known as Arthurs, must embark on a journey to prove their worthiness and determine who is the rightful ruler of Britain.

The narrative primarily follows six main Arthurs, each representing a distinct class and possessing unique abilities. Danchou Arthur, the leader-type Arthur, wields a sword and embodies the classic knight archetype. Renkin Arthur, an alchemist, uses potions and magical concoctions in battle. Yamaneko Arthur, a thief, relies on agility and stealth. Satsujin Arthur, an assassin, specializes in swift and deadly attacks. Kakka Arthur, a mage, commands powerful elemental magic. Ikusagami Arthur, a mercenary, excels in brute strength and combat prowess. Together, they form an unlikely alliance, traveling across the land to confront threats and uncover the truth behind the proliferation of Excaliburs.

The plot unfolds as the Arthurs face various challenges, including battles against powerful enemies, internal conflicts, and moral dilemmas. Their journey is driven by the need to protect Britain from external threats, such as the invading Viking forces, while also addressing the internal chaos caused by the existence of multiple Excaliburs. Along the way, they encounter allies and adversaries, each with their own motivations and connections to the legend of King Arthur.

The series explores themes of leadership, destiny, and the burden of power, as each Arthur grapples with their role and the responsibilities that come with being chosen by Excalibur. The dynamic between the Arthurs evolves as they learn to trust and rely on one another, despite their differing personalities and backgrounds. The story balances action-packed sequences with moments of character development, providing insight into the struggles and aspirations of each Arthur.

As the Arthurs progress on their quest, they uncover deeper mysteries surrounding the origin of the Excaliburs and the true nature of their mission. The narrative builds toward a climactic confrontation that tests their resolve and unity, ultimately determining the fate of Britain and the legacy of King Arthur. The series concludes with a resolution that ties together the central conflicts while leaving room for interpretation regarding the future of the Arthurs and their world.