First loves are messy. While settling in as a shoujo manga editor at the famous Marukawa Publishing House, Ritsu Onodera is quite troubled. Working under the stern and superb Masamune Takano is hard enough as it is. However, Masamune is not only Ritsu's first love from middle school but he also suddenly declares that he will make Ritsu fall for him again. Unknown to them, another editor in the department, Yoshiyuki Katori, is in a relationship with the popular manga artist Chiaki Yoshino. The carefree Chiaki fails to notice, however, that his high school friend—Yuu Yanase—thinks of him as more than a friend. The stoic but caring Hatori will not surrender his love so easily. Falling in love for the first time when you are 30 is certainly troublesome. Shouta Kisa, yet another editor, is going out with 21-year-old Kou Yukina, an art student. Despite Yukina's assurances, Kisa cannot help but doubt whether someone like himself is truly worthy of his younger, "sparkling" boyfriend.

After having to deal with jealousy from his co-workers for working under his father's name, prideful literary editor Ritsu Onodera is determined to establish himself in the industry. To accomplish this, he quits his job at his father's publishing company and transfers to Marukawa Publishing. But instead of being placed in their literary division, Ritsu finds himself working as the rookie manga editor for the Emerald editing department, a team that operates under extremely tight schedules in order to meet deadlines. There, Ritsu is introduced to the infamous editor-in-chief Masamune Takano, a persistent man who strives for results. As it turns out, Takano is actually Ritsu's high school love, and it is the aftermath of that heartbreak has caused Ritsu's reluctance to fall in love again. Now with the two reunited after several years of separation, the reestablishment of their relationship is marked by Takano's vow to make Ritsu say that he loves him again. Sekaiichi Hatsukoi follows three couples that are interconnected within the manga industry, with each being subject to the budding of first love.

Kou Kitamura and Aoba Tsukishima are often at odds—even though their families happen to be close friends and business partners. Although the only child of a sports shop owner, Kou has never been interested in playing baseball. Despite this, he possesses an impressive batting ability honed by frequent visits to the local baseball batting center run by the Tsukushima family. On the other hand, Aoba loves to play baseball and is a star player with exceptional pitching form. However, these two seemingly complete opposites share something very important to them—Wakaba Tsukishima, Aoba's older sister and Kou's destined sweetheart. Admired by the quarrelsome duo, Wakaba often finds herself the catalyst to their never-ending rivalry. But whether or not they realize that they have more in common than either would care to admit, only time will tell. The game of baseball may just be what the pair needs to ultimately overcome their own personal struggles. [Written by MAL Rewrite]

On a mysterious night that seems to last for a year, an ordinary college student continues to chase one of his underclassmen, a girl with black hair—the girl of his dreams. Up until now, he has been relying on a simple plan, which is to calculatingly bump into her every day while making it seem like a meaningful coincidence. However, his efforts remain futile as their relationship is not progressing at all.

Meanwhile, the black-haired girl believes that everything is connected by fate and endeavors to experience as many new things as possible, leaving it all for destiny to decide. While strolling along the lively streets of Kyoto, she discovers that the very beginning of her fateful journey—a book she had as a child—is currently being sold in a second-hand bookstore. Upon knowing this, the college student eyes another opportunity to run into her "by chance": this time, he hopes to get the book before she does and finally grasp the thread of fate that could connect their hearts.

[Written by MAL Rewrite]