In world of fantasy and magic, a young adventurer beset upon a mysterious stranger who immediately gets all familiar with him introduces himself as an old acquaintance he won't remember. He's come to recruit him on a grand quest to save the world from a soon to be returning demon lord and he needs the help of our protagonist and his friends to gather shards of magic that each member either possesses or has hidden away.
It's a rocky journey but they assemble their party of would be heroes one by one as they catch up with each other but it becomes apparent that they know strangely little about the shards or why they have them in their possession to begin with. It is only when one of them invokes the power of their shard in a desperate battle against a demon that someone remembers that they did it to seal away the demon lord and that it left them as timeless wanderers and cost them huge chunks of memories they don't even realize are missing.
Others soon do the same and more and more of these missing memories come back to those who had invoked them until only the protagonist was to invoke his shard which was missing and they learn that the stranger was indeed one of their close friends before though the details remain fuzzy to them. It turns out the stranger had his all along and was orchestrating everything to release the seal in proper fashion.
He leaves and tells them to meet at the ritual site if they want to know the truth and while they separate they surmise that the protagonist would be the one to know the stranger best since he's the one whose memories remains inconsistent with the rest. When they meet again, they manage to fill in the gaps who he might be and tells him he doesn't need to do this but he forces them to fight him and release the last shard.
The protagonist regains memories of his brother and what he did to buy them time to truly vanquish the demon lord.