"Free!" centers around a group of friends who share a deep connection through their love of swimming. The story begins with Haruka Nanase, a talented swimmer with a natural affinity for water, reuniting with his childhood friends Makoto Tachibana, Nagisa Hazuki, and Rin Matsuoka. The four had been part of a swim team in elementary school but drifted apart after Rin moved to Australia to pursue competitive swimming. Years later, Rin returns to Japan and challenges Haruka to a race, reigniting their rivalry and prompting Haruka, Makoto, and Nagisa to revive their school's swim club at Iwatobi High School.

As the series progresses, the group faces various challenges, both in and out of the pool. Haruka, who initially swims solely for the joy of being in water, begins to explore competitive swimming more seriously. Makoto, the team's anchor and Haruka's closest friend, provides emotional support and stability. Nagisa, the most energetic and optimistic member, often lightens the mood and encourages teamwork. Rin, driven by a desire to prove himself, struggles with feelings of inadequacy and the pressure to succeed, leading to moments of tension and growth.

The narrative expands as new characters are introduced, including Rei Ryugazaki, a former track athlete who joins the swim club to improve his physical fitness but eventually develops a passion for swimming. The team also encounters rivals from other schools, such as Sousuke Yamazaki, Rin's childhood friend and fellow swimmer, who adds complexity to the dynamics between the characters. The series explores themes of friendship, rivalry, self-discovery, and perseverance, with each character confronting personal challenges and striving to achieve their goals.

Throughout the series, the group participates in various swimming competitions, showcasing their growth as individuals and as a team. The relationships between the characters evolve, with moments of camaraderie, conflict, and reconciliation shaping their journey. The story balances lighthearted, comedic moments with deeper, emotional arcs, creating a narrative that appeals to a wide audience. By the end of the series, the characters have not only improved their swimming skills but also strengthened their bonds, leaving a lasting impact on each other's lives.