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

Sighing, our protagonist, Yuuto, put the key to his house that he just took out back into his pocket without using it. Upon entering, he found his childhood friend, Saotome Ai, fiddling with her phone, relaxing as if it were her own house. The spare key (AIKAGI) to the house was hanging from her neck.

"Ah, welcome back, senpai~."

A girl one year younger than him welcomed him home with a smile, (though she was lazing about while doing so.)

"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 how the love story between our amiable but easy-to-tease protagonist, and his mischievous junior begins.