The world is an isekai fantasy world like any other but in this one the story centers upon a young man who, after being conked on the head, begins remembering a past life adventuring with the world's previous hero nearly two decades ago. 

He died holding off a horde of demons for the hero at the final battle and left behind some lingering regrets. Among them were his orphaned siblings, not being able to pass on their family's sacred rituals and leaving his fiancee hanging.

Compelled to dispel these past regrets, he decides go out into the world to settle them. However, when he bids goodbye to his family he fully awakens his special ability and accidentally uses it on his twin little sisters. 

As luck would have it, this ability of his allows him to pull out a person's past life memories and his lovable little sisters just happened to be there at the final battle with him too - as the Hero and the Demon King. The former Hero also remembers the previous life before that so all three of them aren't very sure how they fit in the present, especially since it's not just the memories alone that are coming back to them. 

Having little choice in the matter, the young man takes responsibility and brings the girls along with him on his journey to settle old scores, thus beginning a sibling outing that has potentially has the fate of the world hanging in the balance.