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.

Evil Shinma (shape-shifting monsters and vampires) roam the Earth on a mission to unleash their darkness upon the Human race. Miyu Royal Princess from the dark is the Chosen One—the one being who must banish the Evil Shinma from the Earth. She has the power to offer Humans the gift of eternal happiness, yet is herself, trapped between two worlds; destined for perpetual solitude and internal conflict. Miyu's only companion is Larva, once an evil Shimna; now her devoted guardian. Together they share a dark journey through the weakness of the human heart and the tragic loss of innocence. Cut off from humanity by the knowledge of what she is, Miyu lives an endless quest as both the hunter and the hunted, on the edge of darkness.

The heavens shake, the thunder rumbles, and Xie Lian appears with an apologetic smile—again! Eight hundred years prior, he was a beloved martial god, known as the Crown Prince of Xianle. Now, he ascends to the heavenly realm for the third time, but simply as a pitiful scrap-collecting god with no followers behind him. On his first mission, Xie Lian finds himself alone in the dark moonlit night. There, a gentle man dressed in red guides him through the forest. However, as abruptly as he appeared, the man suddenly dissipates into a swarm of silver butterflies. Xie Lian later learns that this mysterious stranger was none other than Hua Cheng, the Crimson Rain Sought Flower, a Ghost King feared by both demons and gods alike. But before Xie Lian can figure out why Hua Cheng would help a Heavenly Official like himself, he meets San Lang. A young man possessing great knowledge on not only the Ghost King, but also the now forgotten Crown Prince, San Lang decides to accompany Xie Lian on his journey of unveiling the mysteries of the past. [Written by MAL Rewrite]