Saki Miyanaga is a first-year student at Kiyosumi High School in Nagano Prefecture and the title character of the Saki series. She grew up playing mahjong with members of her family but came to dislike the game because she would lose her otoshidama (New Year's money) if she lost and be scolded if she won. Because of this Saki, developed the ability to end her games with a net score of ±0 in order to avoid winning and losing against them. Over the years, this playing style became a subconscious habit, and she no longer attempts to win a mahjong match.

After her parents separated, Saki remained with her father in Nagano while her older sister Teru moved to Tokyo with her mother. She eventually lost touch with her sister but is aware that Teru has become a top mahjong player in her high school. Upon entering Kiyosumi, Saki is invited by her childhood friend Kyoutarou Suga to visit the school's mahjong club. She reluctantly agrees and proceeds to play several rounds with fellow first-year students Nodoka Haramura and Yuuki Kataoka. Club president Hisa Takei becomes aware of Saki's ability to manipulate her score during the match, to the other members' astonishment.

Both Nodoka and Hisa confront Saki, and she agrees to return to the mahjong club for another match. Saki continues to display the ability to manipulate her score, but Hisa develops a strategy for her to be able to win the match rather end it with no net score. Upon experiencing the excitement of winning a mahjong match against people outside her family, Saki decides to join the club. She grows close to Nodoka and makes a promise with her to reach the national team tournament, where Saki seeks to reconcile with Teru.