He woke up to find himself in another world.

The protagonist, once a Japanese salaryman, awoke to find himself reborn as an infant.

It seemed he had died from overwork and reincarnated into this other world...

Although born into a noble family, his mother's status as a concubine meant his official standing was that of a commoner.

To this protagonist, someone appears whom he wishes to protect with his very life.

His biological younger sister, born from the same mother.

And it seems his sister is equally determined never to let go of her big brother...?

Thus begins the reincarnator's life, dedicated to living with his beloved sister.