"Osamake: Romcom Where The Childhood Friend Won't Lose" centers on Maru Sueharu, a high school student and aspiring light novel author, who harbors feelings for his classmate, Shirokusa Kachi. Kachi, a popular and talented idol, appears to reciprocate his feelings, leading Sueharu to believe they are on the verge of a romantic relationship. However, his hopes are shattered when he discovers Kachi is secretly dating Tetsuhiko Kai, a famous actor and Sueharu's childhood friend. Feeling betrayed and heartbroken, Sueharu confides in his other childhood friend, Kuroha Shida, who has always been by his side. Shida, who secretly harbors feelings for Sueharu, proposes a plan: they will pretend to date to make Kachi jealous and help Sueharu move on.

As the fake relationship progresses, Sueharu begins to see Shida in a new light, while Shida struggles to balance her genuine feelings with the pretense of their arrangement. Meanwhile, Kachi becomes increasingly conflicted about her own relationship with Kai, as she begins to question her feelings for Sueharu. The dynamic between the characters grows more complex as past relationships, hidden emotions, and misunderstandings come to light. The series explores themes of unrequited love, rivalry, and the blurred lines between friendship and romance, as the characters navigate their intertwined relationships and personal growth.

The main characters include Maru Sueharu, the earnest and somewhat naive protagonist; Kuroha Shida, the sharp-tongued yet caring childhood friend; Shirokusa Kachi, the idol with a complicated love life; and Tetsuhiko Kai, the charismatic actor whose presence complicates the relationships. The narrative unfolds through a mix of comedic and dramatic moments, highlighting the emotional struggles and evolving connections between the characters. The story ultimately delves into the challenges of love, loyalty, and self-discovery in the context of high school life and the entertainment industry.