Water Polo Yankees follows Naoya Inaba, a young man who grew up in overseas and returns to Japan with a romanticized, 'yankee' (delinquent) image of what a Japanese high school student should be. He transfers to Kasumino Technical High School, a school on the verge of closing, and decides to revitalize the school's spirit by forming a water polo team, despite the sport being considered the 'delinquent's antithesis.' As Naoya recruits a ragtag group of students, he faces intense rivalry from the elite Suiran High School water polo team. The series explores themes of friendship, personal growth, and the struggle to find one's identity. Through rigorous training and team bonding, the boys learn that true strength comes from dedication and unity rather than the superficial toughness Naoya initially idolized.