In the future, the cities of the Earth merge into a single megalopolis. Life has stabilized, and each new day is similar to the preceding one. Human beings have lost connection to the culture of the past. The huge city doesn't even have a name, and is simply called "City". Poor people live in the slums, and have no access to the quarters of the rich. Corruption reigns. The hero of the game belongs to an organization of assassins known as "Card". His last mission is to kill corrupted policemen who have been trafficking drugs. The hero must hide, but his relationships with other "Card" members are anything but clear...

Four Flush is essentially a visual novel, though it offers more choices of locations and dialogue options that it is usual for games of this genre. There is no fixed command-based interaction. Most of the game progression, despite the presence of location and dialogue choices, is linear. There are a few scenes with nudity and descriptions of sex, though not particularly "hardcore".