Motto! Seieki Tairyou Chuunyuu!

Main character is a baby-faced short boy. He is often picked on by mean girl group because of his looks. One day, he spots a scene where a girl is being picked just like he is. He tries to stop it, however, it turns out they both get into worse situation.

There, a man called Takeda appears.
He hands a medicine to main character. The medicine, made of cum, supposed to make girls into his slave. It's a revenge to those girls. He hands another medicine to a girl, which makes girl into Futanari, so that she can perform her revenge as well. The main character and the heroin consider, and decide to take those medicines to perform their revenge.

[From DLsite English]