Noboru, a college entrance exam candidate, was helping his brother and his wife move their belongings after they got married and moved out of their company housing. There was one thing he was looking forward to. That was living under the same roof as his beautiful and busty sister-in-law, Rin. And with a single word from his brother, Noboru ended up getting tutored by his sister-in-law. Even though she was his sister-in-law, she was the first woman to enter his room, and Noboru's heart couldn't help but beat faster during the private lessons just the two of them. Seeing his brother-in-law respond so obediently in both Kokoro and body, Rin also began to develop a maternal instinct and a mischievous Kokoro...