"She is here again, huh...?"

While sighing, our protagonist, Yuuto, returns the key to his house in his pocket back again. Upon entering, he finds his childhood friend, Saotome Ai, fiddling with her phone, relaxing as if it is her own house. A spare key (AIKAGI) could be seen hanging down her neck.

"Ah, welcome back, senpai~."

A younger girl welcomes him with a smile. ("She's totally settled in, hasn't she?")

"...I'm back. ...Hey, how many times have I told you not to come in as you please? Also, give me back my spare key already."

Revolving around the AIKAGI, this is a love story of our amiable but easy-to-tease protagonist, and his mischievous little junior.