Sono Te wo Totte

Mizuki has to move often because her father's work situation. As a result, she never makes friends and is a lonesome girl. Just after one such move, Mizuki meets a beautiful girl named Yuuko who takes an uncanny interest in her. Mizuki is surprised to find her heart beating faster in Yuuko's presence.

When she uses the schools bathroom, she is attacked by tentacles that come out of the toilet and rape her. Afterwards, she finds herself having sexual feelings and thoughts about Yuuko and an uncontrollable urge to masturbate. Suddenly, Yuuko invites Mizuki to her home...

(Source: Dynasty Reader)