81diver is a Japanese manga and live-action television drama series centered on the game of shogi, also known as Japanese chess. The title refers to the 81 squares on a standard shogi board, which measure nine by nine. The story follows Kentaro Sugata, a young man who devoted twenty years of his life to becoming a professional shogi player but failed to achieve the necessary fourth dan ranking. After this setback, he leaves the professional pathway and begins earning a living by gambling on shogi matches at amateur clubs. For a time, he is undefeated in this underground world.

His life changes when he encounters and loses to a young woman named Soyo Nakashizu, a 19-year-old shogi prodigy known as Akihabara's Champion. Soyo possesses extraordinary shogi skills and holds an undefeated streak of tens of thousands of consecutive victories. Outside of playing shogi, she works part-time as a maid at a cleaning club in the Akihabara district, often appearing at customers' homes in cosplay attire. The defeat to Soyo, combined with his worsening financial situation, shocks Kentaro and reignites his passion for the game. He also discovers her secret identity, which further draws him into her world.

Determined to improve and face Soyo again, Kentaro is guided by her through the hidden world of shin Kenshi, or true shogi masters who gamble with their skills. He recalls advice from his former teacher, who told him to metaphorically dive into the 81 squares of the board. By focusing his mind and uttering the word Diver, Kentaro enters a mental state where he can visualize the board and its possibilities with perfect clarity, allowing him to find seemingly impossible comeback moves. This ability earns him the nickname 81diver.

As the story progresses, Kentaro trains under a series of eccentric and powerful shogi masters chosen by Soyo. These include Nikogami, a homeless but highly skilled elderly player whose name contains two characters for god; Jiro Monjiyama, a popular manga artist who talks to his shogi pieces as if they are alive; and Shito Kirino, a master doll maker who uses a unique resin to craft realistic figures. Each opponent teaches Kentaro different strategies and philosophies about the game. The series also introduces a shadowy organization known as the Kishoukai, a group of amateur players whose goal is to defeat professional shogi players, presenting a larger conflict for the characters. The live-action adaptation aired as an 11-episode drama on Fuji TV from May to July 2008.