Tucked away in the darkest depths of this world, tales of the bizarre and the supernatural quietly unfold. These inexplicable stories are chronicled throughout the pages of a certain strange encyclopedia, sheltered within a crumbling, decrepit building. Do you dare to open its cover and experience the horrors firsthand? Each of the macabre tales held within the book's pages details some unusual, surreal experiences that often come to a gruesome end. A man searches for his adulterous wife, only to find himself at the mercy of otherworldly visitors; a boy befriends a snowman who harbors a sinister secret; crop circles suddenly form on a family farm, created by some unexpected visitors; hidden in plain sight, menacing mechanical beings continue on undetected. In all of these horrifying stories, nothing is as simple as it seems, revealing a terrifying darkness that perhaps might have been best left alone.

The world's greatest assassin had sworn lifelong allegiance to the organization that raised him. However, despite his loyalty, that very same organization takes action to silence him, ultimately leading to his demise. Drowning in frustration and regrets he can no longer suppress, he finds himself in an audience with a goddess attracted by his exceptional skills. The goddess offers him reincarnation into a magnificent world of swords and magic so he can perform a crucial mission: prevent that world's destruction by slaying its hero. Accepting the goddess' request, he is reborn as Lugh Tuatha Dé, the son of a noble family of assassins serving the Alvan Kingdom. Under the guidance of his father, Lugh learns new assassination techniques that significantly differ from the cold-blooded and unsympathetic killing style of his previous life. Furthermore, his other talents bloom, allowing him to meet new allies and acquaintances. Even so, Lugh knows that his efforts are far from adequate, because a monumental adversary such as the hero can only be defeated with perfection.

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.

Struggling yet brilliant 19-year-old Minato Sahashi has failed his college entrance exams for the second time, resulting in him being regarded as worthless by those around him. However, the course of his seemingly bleak future is altered dramatically when a beautiful, supernatural woman falls from the sky and into his life. That woman, Musubi, is a unique being known as a Sekirei, a humanoid extraterrestrial with extraordinary abilities. These aliens are known for kissing humans carrying the Ashikabi gene in order to awaken additional latent powers deep within. Recognizing the potential within the seemingly insignificant youth, Musubi kisses the bewildered Minato, initiating a bond between the two of them. This drags him into the high-stakes world of the Sekirei, where he and his new partner must compete against others in a battle for survival called the Sekirei Plan. However, unbeknownst to the contestants, there is far more at risk that what the competition initially entailed.