"D4DJ First Mix" centers around Rinku Aimoto, a cheerful and energetic high school girl who transfers to Yoba Academy after living abroad. Rinku’s passion for music and DJing leads her to form a DJ unit called Happy Around! with her classmates. The group includes Maho Akashi, a skilled but perfectionist DJ; Muni Ohnaruto, a shy and reserved girl with a talent for art; and Rei Togetsu, a calm and composed upperclassman who plays bass. Together, they navigate the challenges of performing, creating music, and building their identity as a unit.

The narrative explores the dynamics of friendship and collaboration as Happy Around! prepares for performances and competes in DJ battles. Each member brings unique strengths and struggles to the group, with Rinku’s enthusiasm often clashing with Maho’s meticulous nature, while Muni’s insecurities and Rei’s quiet confidence add depth to their interactions. The series highlights their growth as individuals and as a team, emphasizing the importance of communication and mutual support.

As the story progresses, Happy Around! encounters other DJ units, such as Peaky P-key, Photon Maiden, and Merm4id, each with distinct musical styles and personalities. These interactions provide opportunities for collaboration, rivalry, and learning, further enriching the narrative. The series balances lighthearted moments with emotional arcs, particularly in exploring Muni’s journey to overcome her fear of failure and Rei’s past experiences as a former member of another DJ unit.

The plot is driven by the characters’ passion for music and their desire to connect with audiences through their performances. Themes of creativity, perseverance, and self-expression are woven throughout, as the characters strive to create music that resonates with both themselves and their listeners. The series culminates in a climactic event where Happy Around! showcases their growth and unity, solidifying their place in the vibrant world of DJ culture.