"Sanrio Danshi" centers on a group of high school boys who find solace and joy in their love for Sanrio characters, despite societal expectations and stereotypes about masculinity. The protagonist, Kouta Hasegawa, is a reserved and introverted student who secretly adores Hello Kitty. His passion for Sanrio is initially a source of embarrassment, but he gradually learns to embrace it after meeting others who share similar interests. Among them is Yuu Mizuno, a confident and outgoing classmate who loves Pompompurin, and Shunsuke Yoshino, a diligent student council member who is fond of My Melody. The group also includes Ryou Nishimiya, a quiet and artistic boy who admires Cinnamoroll, and Seiichirou Minamoto, a mature and composed senior who has a soft spot for Kiki and Lala from Little Twin Stars.

The narrative explores the boys' daily lives as they navigate friendships, school responsibilities, and personal growth, all while bonding over their shared love for Sanrio. Each character’s backstory is gradually revealed, highlighting how their affection for these characters provides comfort and a sense of identity. Kouta, in particular, struggles with self-acceptance and fears judgment from his peers, but his friendships with the group help him overcome these insecurities. The series also touches on themes of individuality, the importance of hobbies, and the challenges of defying societal norms.

Throughout the series, the boys participate in various activities related to their favorite Sanrio characters, such as visiting Sanrio stores, attending themed events, and creating handmade crafts. These moments are interwoven with their personal journeys, showcasing how their shared passion strengthens their bond and helps them face life’s challenges. The story balances lighthearted, comedic moments with heartfelt, emotional scenes, creating a narrative that resonates with viewers who appreciate themes of self-discovery and camaraderie.