After a peculiar reincarnation into a mystical realm as an Immortal, Sansui Shirokuro faces the daunting task of honing his immense powers over centuries. However, his routine existence takes a surprising turn when he is entrusted with raising an infant by his master. Along this journey, he crosses paths with a proud noblewoman and her spirited bodyguard. Despite his unassuming swordsmanship, the world is about to witness the remarkable skills a swordsman can attain after five hundred years of dedication.