"Princess Nine" is a 26-episode anime series that centers on the formation and struggles of an all-girls baseball team at Kisaragi Girls' High School in Tokyo. The story begins with Ryo Hayakawa, a talented pitcher with a natural gift for baseball, discovered by Keiko Himuro, the chairwoman of Kisaragi Academy. Keiko, a former Olympic softball player, aims to challenge the male-dominated world of high school baseball by assembling a team of girls to compete in the national high school baseball tournament, the Koshien.

Ryo, the daughter of a deceased professional baseball player, initially resists joining the team due to her humble background and the societal stigma surrounding girls in sports. However, her passion for baseball and Keiko's determination inspire her to take on the challenge. The series follows the recruitment of eight other girls, each with unique personalities and skills, as they come together to form the Kisaragi Girls' Baseball Team. The team includes Izumi Himuro, Keiko's daughter and a skilled catcher; Yuki Ochiai, a disciplined and serious shortstop; and Koharu Hotta, a cheerful and energetic outfielder, among others.

As the team trains and competes, they face numerous obstacles, including societal prejudice, internal conflicts, and the immense pressure of proving themselves against established male teams. The narrative explores themes of perseverance, teamwork, and breaking gender barriers, as the girls strive to overcome their limitations and earn respect in a traditionally male-dominated sport. The series also delves into the personal lives of the characters, highlighting their individual struggles and growth, such as Ryo's journey to reconcile her past and embrace her potential as a leader.

The climax of the series revolves around the team's efforts to qualify for the Koshien tournament, culminating in a high-stakes game against a formidable male team. The outcome of their journey is left open-ended, emphasizing the importance of their determination and the impact they have made rather than a definitive victory. "Princess Nine" combines sports drama with character-driven storytelling, offering a compelling narrative about ambition, resilience, and the pursuit of equality in the face of adversity.