Saki Miyanaga is a skilled mahjong player, but she hates the game since she would be punished by her family whether she won or lost when she played with them. Upon entering high school, Saki's friend, Kyōtarō Suga, takes her to their school's mahjong club so Saki can be a fourth player. There, she meets Nodoka Haramura, a very skilled mahjong player and previous junior high champion, and Yūki Kataoka, an energetic first-year who is skilled in mahjong though has trouble staying focused. Saki only plays a few hands and leaves shortly after the club president and student council president, Hisa Takei, wakes up. Hisa notices Saki managed to keep her score at zero, a difficult feat. Hisa manages to get Saki to come back the next day to play a couple more games with Nodoka, Yūki, and Mako Someya, a second-year member of the club. Saki shows her skill in mahjong by attaining the zero score in the first game.

Hisa announces to the club that they are now signed up for the national qualifying tournament, and she hands out the rules. Yūki sees that one school, Ryūmonbuchi, are extremely skilled and even won the tournament the previous year despite being all first-year students. Hisa asks Saki and Nodoka to help out at Mako's family's cafe, telling them that learning about the real world is also important to improve their mahjong skills. When they arrive at the cafe, they see that it is actually a maid cafe, and they have to wear embarrassing maid uniforms. Patrons can also play mahjong there, and Saki and Nodoka easily win against two customers. After one leaves, a woman who Saki is intimidated by joins them. The woman completely mops the floor with them, and at the end it is revealed that she is Fujita, a professional mahjong player. Saki and Nodoka later find out that Hisa set the whole thing up so that they could also feel what real defeat is like. Hisa explains that the club is going on a training camp to improve their skills.

As the round continues, Saki finally recognizes Koromo's aura. She starts putting pressure on her opponents by reducing their tenpai probabilities and playing two haitei raoyue moves in a row. However, this is broken up when Yumi allows Saki to win with a her discard. Yumi then follows this up by getting some points from Kana.

During the break, Mihoko goes to comfort Kana, and Nodoka catches up with Saki to reaffirm their promise and rebuild her confidence. Fujita warns Koromo to play mahjong properly.

Remembering the pact she had made with Mihoko, Kana gets a burst of confidence and regains a lot of points. Yumi also gets a good play, and Koromo, having paid all her attention to Saki, gets a little agitated. Saki takes off her shoes and socks, remembering the times she played mahjong at home and at the training camp.

Koromo recalls that when her parents died, no matter how many times Touka brought her friends, she still always felt lonely, believing her only worth is in mahjong. Koromo's powers cause a brief blackout, but Saki remains calm throughout. Koromo gets another haitei raoyue once the game resumes, leaving one more hand. Koromo is a bit shocked when Saki says she is having fun playing against her.

Kyōtarō tells Saki about an upcoming summer festival and she invites the rest of the club to go too. Hisa sends out invitations to the other schools to join them for Kiyosumi's training camp for the nationals. Yūki ends up flunking her math midterm and has to take a makeup test, failure of which means she will not be able to attend the camp, so Saki and Nodoka decide to help her study. Hisa prohibits them from using the club room and they end up having trouble finding other places to study. Ryūmonbuchi and Tsuruga send their responses for the camp, while Mihoko delivers hers to Hisa personally. After Yūki's makeup test, the club members go to the festival, and Hisa watches from afar with Mihoko. Yūki passes her makeup test and all the schools go to the training camp.