Just like in the anime, Yuuri Shibuya got flushed down the toilet out of his normal, average life into an unknown world, thus being appointed as the new Maou (Demon King). The game has three "events" -- the festival, the dance/party and something concerning Gwendal.
Like most games, you choose your own path therefore you choose your own ending. You play Yuuri and the characters you are going to be with most of the time would be Wolfram, Conrart, Gunter and Gwendal depending on whom and what you choose.
It's a Japanese game but you could still make your way through the game because it doesn't have any mission for you to do and stuff. All you need to do are simple clicking, watching and enjoying.
Summary by ding1989