On a winter day, a university student named Ittetsu meets a small black cat in the garbage dump and decides to take it home, naming it Kotetsu and making it his new family member.

The two of them live peacefully, and one year later, Kotetsu has grown "big" beyond Ittetsu's imagination... into an attractive woman! Ittetsu tries various strategies to resist his urges towards Kotetsu, but Kotetsu, who loves Ittetsu, actively seduces him without any concerns!

As their hearts and desires clash due to their concern for each other's happiness, will they be able to overcome the family boundaries and become a couple?

Note: Includes a free voice comic if bought on DLsite.