"R-15" centers on Taketo Akutagawa, a 15-year-old prodigy novelist who enrolls in the prestigious Inspiration Academy, a school for gifted individuals. Taketo specializes in writing risqué adult novels, a talent that often lands him in awkward and humorous situations due to his overactive imagination and tendency to misinterpret everyday interactions. His works are deeply personal, often reflecting his own fantasies and desires, which he struggles to keep separate from his real-life experiences.

The series explores Taketo's life at Inspiration Academy, where he encounters a diverse group of classmates, each with their own unique talents and quirks. Among them is Fukune Raimu, a shy and reserved girl who excels in calligraphy but harbors a secret identity as a popular idol. Raimu becomes a central figure in Taketo's life, as her presence often inspires his writing while also complicating his personal relationships. Other notable characters include Raika Hoshino, a brilliant inventor with a penchant for creating eccentric gadgets; Utae Sonokoe, a talented singer with a sharp tongue; and Veronica, a stoic and mysterious transfer student who adds an air of intrigue to the group.

As Taketo navigates the challenges of school life, he frequently finds himself entangled in comedic and sometimes surreal scenarios, often triggered by his vivid imagination. His adult-themed writing frequently causes misunderstandings, leading to humorous confrontations with his peers and teachers. Despite these challenges, Taketo remains determined to pursue his passion for writing, often drawing inspiration from the people and events around him.

The narrative balances comedy, romance, and lighthearted drama, with a focus on the interactions between Taketo and his classmates. The series delves into themes of creativity, self-expression, and the complexities of adolescence, as the characters grapple with their talents, aspirations, and personal insecurities. While the tone is predominantly comedic, there are moments of introspection and character development, particularly as Taketo learns to reconcile his imaginative world with the realities of his relationships.

"R-15" presents a blend of humor, romance, and school-life dynamics, with a cast of eccentric characters whose interactions drive the story forward. The series offers a lighthearted yet occasionally poignant exploration of the challenges and joys of being a young creative prodigy in a competitive and unconventional environment.