Seven years before the events of the second season of Liar Game, the prequel miniseries Liar Game: Episode Zero explores the formative experiences that shaped three of its central figures: Nao Kanzaki, Shinichi Akiyama, and Norihiko Yokoya. The narrative is split into three distinct segments, each set in 2002, revealing the crucial events that set them on their eventual collision course with the Liar Game Tournament.

The first segment, focusing on a thirteen-year-old Nao Kanzaki, illustrates the roots of her famously honest nature. After being deceived by an elderly man who borrowed money for a school festival fund and then disappeared, a disillusioned Nao encounters Koichi Eto, a young man with dreams of becoming a singer who has himself been scammed. Later, Eto attempts to rob the Kanzaki home, but Nao catches him. Instead of turning him in, she tells him she does not believe he is a bad person and urges him to flee. Moved by her naive trust, Eto abandons his theft and leaves behind a guitar pick, which Nao treasures as a good luck charm. The next day, the old man unexpectedly returns, repaying his debt, reinforcing Nao’s belief in trusting others.

The second segment follows a young Shinichi Akiyama, already known at university for his brilliant mind. He returns home to find his mother has been pressured into buying a fraudulent water purifier. Recognizing the scam from a friend’s experience, Akiyama confronts the salesman. He exposes the trick by pointing out that the supposedly filtered water was actually coming from a hose hidden in the salesman's sleeve. Having turned off the house’s main water supply before the demonstration, Akiyama proves the device is a fake and forces the salesman to return the money.

The third and most pivotal segment details the rise and fall of Norihiko Yokoya. As the head of a successful multi-level marketing company called Road Free, Yokoya has earned immense profits for his superior, the mysterious Hasegawa. Seeking to break free from being treated as a subordinate, Yokoya invests his entire organizational fund into a new venture proposed by an associate. This company, however, is a dummy corporation created by Shinichi Akiyama as an act of revenge. Akiyama’s mother, having been driven into debt by Yokoya’s schemes, has committed suicide, and Akiyama masterminds the bankruptcy of Road Free as retribution. Following the collapse, Yokoya fakes his own death using a stand-in. A police officer named Tanimura suspects the truth and the involvement of a powerful secret organization behind the multi-level company. When his investigation is suppressed by the LGT Secretariat, Tanimura is forced out of the police force, eventually leading him to join the very organization he once tried to expose.