Gitai Saimin

Murakoshi Shinta, a student, conceals a tendency toward vivid delusions while leading a bleak and uneventful life. An alien parasite with tentacles eventually assumes control of his penis. When Murakoshi protests that his exposed groin prevents him from going outside, the creature informs him its mind-clouding power will render him unnoticeable. He proceeds to employ this ability to fulfill his own desires.