Sixteen years after the "Law for Public Order and Morals in Healthy Child-Raising" banned coarse language in the country, Tanukichi Okuma enrolls in the country's leading elite public morals school and is soon invited into the Anti-Societal Organization (SOX) by its founder, Ayame Kajou. As a member blackmailed into joining by Ayame, Tanukichi ends up taking part in obscene acts of terrorism against the talented student council president Anna (for whom Tanukichi has a crush on).
Learn more about the world of Shimoneta to Iu Gainen ga Sonzai Shinai Taikutsu na Sekai! (Wow, that title's a mouthful. Heh)
Here's an interview with the original creator of ShimoSeka, author Hirotaka Akagi.