"Sekaiichi Hatsukoi" is a manga series that explores the professional and romantic lives of individuals working in the publishing industry, particularly within the shōjo manga department of a fictional publishing company, Marukawa Shoten. The narrative intertwines multiple story arcs, each focusing on different characters and their relationships, while maintaining a central theme of love and self-discovery in a workplace setting.

The primary arc centers on Ritsu Onodera, a talented editor who leaves his father's prestigious publishing company to prove his capabilities independently. He joins Marukawa Shoten's shōjo manga department, only to discover that his new boss, Masamune Takano, is his high school love interest. Takano, now a demanding and perfectionist editor, recognizes Ritsu immediately, though Ritsu initially fails to recall their past connection. Their professional relationship is fraught with tension, as Takano pushes Ritsu to excel while harboring unresolved feelings for him. Over time, their shared history resurfaces, leading to a complex dynamic that blends rivalry, respect, and rekindled romance.

Parallel storylines explore the lives of other characters within the same workplace. One arc focuses on Yoshiyuki Hatori, a senior editor, and his relationship with Chiaki Yoshino, a manga artist. Their bond evolves from a professional partnership to a deeply personal connection, navigating challenges such as creative differences and personal insecurities. Another arc delves into the relationship between Kisa Shouta, a reserved and introverted editor, and Yukina Kou, a flamboyant and confident manga artist. Their contrasting personalities create a compelling dynamic, as they learn to understand and support each other both professionally and romantically.

The series also introduces supporting characters who contribute to the overarching narrative, such as Yuu Yanase, a playful and mischievous editor, and his interactions with his colleagues. Each character's journey is interwoven with themes of ambition, vulnerability, and the pursuit of love, set against the backdrop of the competitive and fast-paced world of manga publishing.

"Sekaiichi Hatsukoi" balances humor, drama, and romance, presenting a nuanced portrayal of relationships that develop in a professional environment. The series emphasizes the challenges of balancing personal and professional lives, while exploring the complexities of love and identity. Through its interconnected storylines, the manga offers a multifaceted look at the lives of individuals navigating both their careers and their hearts.