Life is idyllic and unassuming in the small town of Sotoba, a simple place where everyone knows everyone. However, tragedy strikes when Megumi Shimizu, a young girl with high aspirations, unexpectedly passes away from an unnamed illness. Over the torrid summer months, as more unexplained deaths crop up around the village, the town's doctor—Toshio Ozaki—begins to suspect that something more sinister than a mere disease is at play. Toshio teams up with Natsuno Yuuki, an apathetic and aloof teenager, and siblings Kaori and Akira Tanaka, two of Megumi's friends, to unravel the dark mystery behind the deaths in Sotoba. With their combined efforts, the investigation leads them toward an eerie secret pertaining to the new family in the Kanemasa mansion.

It has been one year since Tsukune Aono enrolled at Youkai Academy, and since then his life has taken an interesting turn. In addition to being the only human at a school for monsters, he has attracted a bevy of beautiful women who want him all for themselves: the sexy succubus Kurumu Kurono; Yukari Sendou, a witch; stalker and yuki-onna Mizore Shirayuki; and Moka Akashiya, a kind vampire who, when her rosary is removed, reveals a darker personality. But soon, trouble visits Youkai Academy in the form of Moka's younger sister, Kokoa Shuzen. She is furious that her true older sister is sealed within Moka's rosary, and vows to bring out Moka's darker self. However, Kokoa's thoughtless actions may affect more than just her sister...

The Holy Grail is a powerful, ancient relic capable of granting any wish the beholder desires. In order to obtain this power, various magi known as masters summon legendary Heroic Spirits called servants to fight for them in a destructive battle royale—the Holy Grail War. Only the last master-servant pair standing may claim the Grail for themselves. Yet, the third war ended inconclusively, as the Grail mysteriously disappeared following the conflict. Many years later, the magi clan Yggdmillennia announces its possession of the Holy Grail, and intends to leave the Mage's Association. In response, the Association sends 50 elite magi to retrieve the Grail; however, all but one are killed by an unknown servant. The lone survivor is used as a messenger to convey Yggdmillennia's declaration of war on the Association. As there are only two parties involved in the conflict, the Holy Grail War takes on an unusual form. Yggdmillennia and the Mage's Association will each deploy seven master-servant pairs, and the side that loses all its combatants first will forfeit the artifact. As the 14 masters summon their servants and assemble on the battlefield, the magical world shivers in anticipation with the rise of the Great Holy Grail War.

The day is as normal as it can be in high school as the students peacefully go about their everyday activities until an unprecedented catastrophe strikes the school, killing every person in its wake. Guided by what seems to be a miracle, a handful of students are fortunate enough to be reincarnated into another world as nobles, princes, and other kinds of people with prestigious backgrounds. One girl, however, is not so lucky. Being reborn as a spider of the weakest kind, she immediately experiences the hardships of her dire situation. Even so, she must press on to survive the numerous threats that endanger her life. Discovering that her new world has a system like that of an RPG, she tries her best to hunt prey and defeat monsters to level up and evolve. As she gradually grows stronger, she hopes one day her efforts will be rewarded, and that she will be granted a better life.