"High Speed! -Free! Starting Days-" serves as a prequel to the "Free!" series, exploring the formative years of its main characters during their time at Iwatobi Elementary School. The narrative centers on Haruka Nanase, a boy with an innate passion for swimming, particularly the freestyle stroke. His talent and love for the water set him apart, but he struggles to find a sense of purpose or connection with others through his abilities. This changes when he meets Makoto Tachibana, a kind and supportive classmate who becomes his closest friend and swimming partner. Together, they form a bond that revolves around their shared love for the sport.

The story gains momentum with the arrival of Rin Matsuoka, a transfer student from Australia who is equally passionate about swimming. Rin’s competitive nature and determination to excel challenge Haruka, sparking a rivalry that evolves into a complex friendship. The trio, along with their classmate Nagisa Hazuki, forms a relay team, aiming to compete in a local swimming tournament. Their journey highlights themes of camaraderie, personal growth, and the pursuit of individual and collective goals.

As the narrative progresses, the dynamics between the characters deepen. Haruka’s introspective nature and focus on the sensation of water contrast with Rin’s driven and ambitious personality, creating a compelling interplay of motivations. Makoto acts as a stabilizing force, balancing the group with his empathy and leadership, while Nagisa brings enthusiasm and humor, lightening the tone of their interactions. The series delves into their struggles, both in and out of the pool, as they navigate challenges such as self-doubt, rivalry, and the pressure to perform.

The tournament serves as a pivotal moment, testing their teamwork and individual resolve. The outcome of the competition not only shapes their relationships but also sets the stage for their future paths in swimming. Through its exploration of friendship, competition, and personal identity, "High Speed! -Free! Starting Days-" provides a foundational backstory for the characters, offering insight into the origins of their motivations and connections that define the broader "Free!" series. The narrative remains grounded in its focus on character development, emphasizing the emotional and psychological growth of its protagonists as they discover the meaning of swimming in their lives.