Arthur Pendragon, heir of House Pendragon and would-be king of sixth-century Camelot, faces civil war. His claim to the throne remains unproven because he cannot withdraw the legitimacy-granting sword Excalibur from its stone. Guarded by Logne, a time-traveling dragon serving his lineage, Arthur contends with rival factions destabilizing his rule.

When Logne transports manga artist Marina Ikeda and her friends from modern Tokyo to Camelot and subsequently dies, Arthur meets them. Marina reveals crucial information: Arthur's success in extracting Excalibur depends on possessing the sword's sheath and timing the attempt with an impending solar eclipse. This knowledge becomes vital for uniting the kingdom and preventing civil war.

Arthur's authority is directly challenged by Lady Dola, who seeks the throne. She attempts to sabotage his destiny by stealing Excalibur's sheath and disrupting the eclipse ritual. During the eclipse, Lady Dola confronts Arthur and Marina in a demonic form, aiming to kill them and seize Excalibur. Arthur's fate hinges on this confrontation, where reclaiming the sheath and fulfilling the prophecy is essential to secure his kingship and bring peace to Camelot.