"Mach Girl" centers on a high school student named Sora, who leads a double life as a competitive racer in the underground world of high-speed motorcycle racing. By day, Sora is an unassuming and diligent student, but by night, she transforms into "Mach Girl," a fearless and skilled rider known for her unmatched speed and precision. The series explores the challenges Sora faces in balancing her ordinary life with the adrenaline-fueled demands of her secret identity.

The narrative begins with Sora inheriting a custom-built motorcycle from her late father, a former racer who inspired her passion for speed. Determined to honor his legacy, she enters the dangerous and male-dominated world of underground racing. The races are intense and high-stakes, often involving rivalries, sabotage, and life-threatening risks. Sora’s journey is marked by her struggle to prove herself in a world where she is constantly underestimated due to her age and gender.

Key characters include Ren, a seasoned racer who becomes Sora’s mentor and ally, and Kaito, a childhood friend who unknowingly becomes entangled in her secret life. Antagonists such as the ruthless racer Ryoma and the enigmatic organizer of the underground races, known only as "The Boss," add tension and conflict to the story. The series also delves into Sora’s relationships with her classmates and family, highlighting the emotional toll of her dual life.

As the story progresses, Sora faces increasingly difficult challenges, both on and off the track. Her determination to win races is driven not only by her love for racing but also by her desire to uncover the truth behind her father’s mysterious death. The series builds toward a climactic showdown where Sora must confront her past, outrace her rivals, and solidify her place as a legend in the underground racing world. The narrative balances action-packed racing sequences with moments of personal growth and introspection, creating a compelling and multifaceted story.