"Rikka, you brought this upon yourself, y'know?"

Childhood friends Rikka and Naoto were next door neighbors and have known each other since they were young.

She still goes over to his place to wake him each morning, and the two have continued their childhood friend relationship even after Naoto enrolled in university and began living by himself, but...
When Rikka sees Naoto bringing girls back to his place, she begins to eavesdrop on them.

And ever since Naoto found out......