A daughter of the chair of a local high school, Yū Honjō, has been kidnapped by someone unknown. Unable to go to the police, the girl's sister, Mai, hires a private investigator to find out what happened to her. The only piece of evidence is a photograph from the night of Yū's disappearance, where the missing girl and three other students from the same school are playing a game of mahjong.

Jantei Monogatari is a hybrid of a traditional-style Japanese adventure game and a regular mahjong game. The player takes the role of the unnamed private investigator, and moves from place to place via menus, talking to different characters. The investigation happens through playing mahjong with each suspect: win and they'll tell you a bit more of what they know. At first, the investigator can only speak with the three girls in the photo, but as he wins games of mahjong against them, he gains information and experience that allow him to find new clues and talk to more suspects. The investigator will need to play against a dozen different opponents in his attempt to find out what happened to the kidnapped girl.