The Isle of Koumi, a beautiful island in the Pacific Ocean. On the island, people pass an old legend down from generation to generation that there was the Seabed Palace, an ancient ruin at the bottom of the sea, where the treasure of 2 female pirates, Anne Bonnie & Mary Reed (who really existed 300 years ago), was left. When Conan and his friends visit Koumi Island while on vacation, they meet some treasure hunters. There was something suspicious about them.

Shirou Emiya finds himself an unwilling participant in a deadly competition where seven Mages summon heroic spirits as servants to duel each other to the death. They compete for the chance to make a wish from the Holy Grail, which has the power to grant any wish. Shiro is unskilled as a mage and knows nothing of the Holy Grail War, but he and his servant, Saber, enter into a temporary partnership with another Mage, Rin Tohsaka. However, problems arise between Shirou and Rin’s servant, Archer, who seems to seriously despise him.