The protagonist of the game, a young Japanese male, visits a small town and witnesses two thugs assaulting a terrified young woman. Unable to help her, the hero leaves the town and forgets about the incident. Two years later, he is back in that town, but shortly after having arrived, he is attacked on the street, and robbed of all his possessions. Now he has to get to the bottom of the mystery, but first he needs money. Since the town is big on mahjong tournaments, the hero decides to get his money back by playing that game, at the same time investigating the townspeople... in more than one way. Jan Jaka Jan is a mahjong game in its core, the majority of the gameplay dedicated to mahjong matches (two- as well as four-people) against a variety of opponents. The player can navigate the protagonist around the top-down town and enter various locations, which are viewed from first-person perspective. The player is also able to buy items to increase his parameters during mahjong matches and encounters of sexual nature. Cut scenes occur before and after mahjong games, and many matches and other events are dictated by the story. Defeating a female mahjong opponent usually leads to cut scenes that depict nudity and sexual situations.