"Saki Achiga-hen episode of side-A" is a spin-off of the original "Saki" series, focusing on the Achiga Girls' Academy mahjong team as they strive to compete in the national high school mahjong tournament. The story centers on Shizuno Takakamo, a reserved and observant girl who reunites with her childhood friend, Ako Atarashi, after transferring to Achiga. Ako, a passionate and energetic mahjong player, convinces Shizuno to join the school's mahjong club, which had been disbanded years earlier due to a lack of members. Together, they revive the club and recruit additional members: Kuro Matsumi, a skilled player with a calm demeanor; Yuu Matsumi, Kuro’s younger sister who excels at defensive play; and Arata Sagimori, a transfer student with a mysterious past and exceptional mahjong abilities.

The team’s goal is to qualify for the national tournament, a feat that requires them to overcome formidable opponents in regional and prefectural competitions. Each member brings unique skills and strategies to the table, and their growth as players is a central focus of the narrative. The series delves into the intricacies of competitive mahjong, highlighting the psychological and tactical aspects of the game. Key rivals include teams from elite schools such as Senriyama and Shiraitodai, whose members possess extraordinary abilities that challenge Achiga’s resolve.

The story also explores the personal motivations and backstories of the main characters. Shizuno’s quiet determination and Ako’s fiery enthusiasm drive the team forward, while Kuro and Yuu’s sibling dynamics add depth to their interactions. Arata’s connection to the original "Saki" protagonist, Saki Miyanaga, is gradually revealed, tying the spin-off to the broader narrative of the franchise. The series balances intense mahjong matches with moments of camaraderie and character development, creating a compelling blend of sports drama and interpersonal relationships. Through teamwork and perseverance, the Achiga team aims to prove themselves on the national stage, making their journey a central theme of the series.