One rainy day, on his way home, the protagonist, Sasahara Hiroto, spotted his classmate Minamiya Ria holding an umbrella over an abandoned cat. Ria was a quiet girl who usually avoided interactions with others. Yet here she was, chatting to a cat with childlike innocence.

Hiroto called out to her, suggesting that they take the cat in. Immediately, for some reason, Ria closed in on him and whispered softly,

"I knew it. You are my master, arent you?"

Then, she kissed Hiroto.

From that day on, Hiroto became Rias "master." It was the beginning of sweet and exciting days. But Rias unrestrained advances were beyond Hirotos wildest imagination...!?