Fukubiki! Triangle: Miharu After

Miharu Shinonome has a perfect body.

She is very knowledgeable about sexual activities and often takes the lead with her boyfriend Ushio Irifune.

In a sudden twist of fate, a horrible accident sends her to the hospital, making Ushio and her sister Futaba extremely concerned about her current state of health.

Miharu wakes up losing her memories of Ushio, reverting to her state before she met her boyfriend—shy, moody, and utterly innocent.

Moreover, taking advantage of her sister's memory loss, Futaba seduces Ushio as she also harbors feelings for him.

Ushio needs to help Miharu regain her memories, all the while trying to remain unfazed by Futaba's lustful seduction.