"Ah~, if my life was a game, I want to restart it from this morning!"

What Kazuki received from his parents via the post was a 'little sister'!? According to his parents there were circumstances behind Sabrina...

"A mafia's hidden child?"

"Si."

"And an amateur assassin?"

"Si."

Sabrina hugged tightly onto Kazuki.

"And... I'm your little sister. I wanted to meet you, Onii-chan."

What would happen to a middle school student when he suddenly gets a cute little sister? And that little sister happened to be a blond foreigner with blue eyes? And that little sister happened to be an amateur assassin?

What would happen to our protagonist- Kazuki Outaki's everyday life!