The demon lord of the world has died and been reborn as a human boy after his death. A new demon lord is set to come forth in due time so, bound by his memories of his old life, sets out on a journey to find his successor so he can give them guidance for the coming future. That's when makes the encounter of this lifetime - the new hero of the humans is summoned before him by happenstance.
The ritual had gone wrong and a girl from modern earth falls out of the sky and into the lake he was bathing in. Not realizing who she was, he rescues her from drowning and helps this unusual stranger gain their bearings. Knowing what her arrival meant for this world, he tries to leave her to fate but upon seeing how helpless and afraid she was he decides to help her out and take her to the scholar city to see if they could help her find a way home. They set off and come to know each other over the next few weeks until they reach the city where the pair are captured and taken to the capital where they force her to take up the hero's holy sword so she can inherit the wills and powers of its past owners.
Shadowed by the past heroes, she resigns herself to this new role in the coming war. On the night of celebration she thanks him for all the help and tells him to complete his journey to "see the world" without her but refuses. He confesses to her about who he really is and who he's trying to find. The hero will in her compels her to strike him down but she resists enough to hear him out.
The reason he wants to find the new demon lord is to convince them to broker a peace instead. The previous hero had persuaded him that peace was worth a shot at the very end of their battles. She remains conflicted because of the phantom hate she inherited and nearly stabs him. Facing down the cursed edge he declares to her that she's not a hero but a scared girl who he promised to help get back home. Thus begins their journey in earnest.