The protagonist, Sanjou Tsunsuke, is an ordinary student attending an ordinary school.
His athletic ability, grades, and looks are all average, and he seemed to be living an ordinary life. But one day, all that changed when his young stepmother showed up.
His father, a folklorist, had returned home from a field trip to a certain country, bringing home a woman as his fiancee.

Accustomed to his father's eccentric behavior, Tsunosuke quickly accepted that, but he couldn't hide his confusion when it came to his stepmother, Ami, who is almost the same age as him.
The reason is... she is just his type.
Tsunsuke's heart continues to be tickled by Ami's innocent personality, which, unaware of Tsunsuke's feelings, tries to get along with him because she's his stepmother.

To make matters worse, Ami's older sister, Ruka, arrives.
Ruka has a personality that's the complete opposite of her sister, but she is sexually open and, like a cat, becomes attached to Tsunsuke.
Between the two, Tsunsuke's heart wavers, and gradually he begins to lean towards sexual desire.

His father, who should be able to stop it, is so busy that he hardly ever comes home despite being newlyweds.

The innocent Ami and the seductive Ruka...

Will Tsunsuke be unable to shake off the desire in his heart and end up making a move on her?