College student Nao Kanzaki, whose defining trait is a foolish honesty, one day receives an unexpected package containing one hundred million yen and a letter inviting her to participate in the Liar Game. The rules are deceptively simple: she must compete against another player to swindle the other out of their one hundred million yen within thirty days, using any means necessary. After the time limit, the organization running the game reclaims the original one hundred million yen from each player, meaning the winner keeps the amount taken from the opponent as a prize, while the loser is left with a debt of one hundred million yen. Before she can fully comprehend the situation, Nao naively entrusts her money to her opponent, a former teacher, and is promptly cheated out of the entire sum.

Desperate and facing a life-ruining debt, Nao seeks help from Shinichi Akiyama, a brilliant former con artist and criminal psychology graduate who has just been released from prison for single-handedly bankrupting a massive fraud corporation. Akiyama reluctantly agrees to help Nao, using his genius-level strategic mind and deep understanding of human psychology to turn the tables on her opponent. Their success forces them to continue advancing through the tournament's escalating rounds, as they aim not only to survive but to dismantle the Liar Game organization from within.

The setting is a shadowy, closed-room environment controlled by the Liar Game Tournament Office, an organization whose masked employees design intricate psychological warfare scenarios for the contestants. The narrative follows Nao and Akiyama through a series of progressively complex games. Early rounds include the Minority Rule, where players vote on a binary question, and the minority faction wins, forcing participants to constantly outthink the larger group. Later games, such as the Contraband Game and the musical chairs-themed game, introduce larger casts of recurring antagonists, including the cold and dictatorial Norihiko Yokoya, the manipulative and calculating Yuji Fukunaga, and the cult-like leader Takashi Harimoto. Throughout these arcs, the overly trusting Nao learns to navigate a world built on deceit, often serving as the emotional and moral counterweight to Akiyama's cynical pragmatism, as they work to free their opponents from debt and uncover the true purpose of the sinister tournament.