The Magical Kingdom of Klyrode summons hundreds of heroes from other worlds every year to fight in their war against the Dark One and his army of powerful demons. Banaza is one of those heroes, summoned from the Royal Capital Paluma, but something's not right—Banaza is only an average merchant. He has no magic, no fighting ability, and his stats are abysmal. Worse, a mishap leaves him unable to return home! Rejected as a hero and stranded in another world, abandoned to the far reaches of the kingdom by a cruel king who just wants him gone, Banaza's fate looks pretty bleak. But what will happen once the failed hero candidate finds himself with super cheat powers once he hits level two?

Homeless and fresh out of employment, Mito Yamamoto faces familiar loneliness and despair. Luck is simply not on Mito's side; when he stumbles upon a cafe and cuts his hand, a strange server named Ruka Saotome licks the blood from his wound and even dramatically retches at its apparent disgusting taste. Offended and dispirited, Mito runs off to a bridge, hoping to put an end to his misery. But when Mito accidentally slips, he is miraculously saved by Ruka, who reveals he is a vampire. Unable to drink blood from just any woman for mysterious reasons, Ruka proposes that Mito become his thrall—a vampire's human servant and primary source of food. In this deal, Ruka will shower Mito with love to sweeten his blood, and Mito will gain a warm place to call home. However, there is an issue: Mito is a girl! Concealing her long, flowing hair and feminine body, Mito readily takes this chance and enrolls in the Hijirigaoka Boys' High School while working part-time at the cafe with Ruka. But the more Mito's bond with Ruka deepens, the more she struggles to hide her secret.

After the destruction of their hometown, childhood friends Kafka Hibino and Mina Ashiro make a pact to become officers in the Defense Force—a militarized organization tasked with protecting Japan from colossal monsters known as "kaijuu." Decades later, the 32-year-old Kafka has all but given up on his dreams of heroism. Instead, he cleans up the remains of the slaughtered kaijuu after they are defeated by valiant soldiers—including Mina, who has successfully achieved their shared goal. Upon meeting his new coworker, Reno Ichikawa, Kafka faces a mirror of his past self: an ambitious young man whose one desire is to fight as a member of the Defense Force. Unfortunately, the two are soon involved in a freak encounter with a rogue kaijuu. Though Kafka demonstrates his innate heroic nature and rescues Reno from certain doom, he is left gravely injured. While both men recover in a hospital, Kafka is seemingly attacked by another one of the beasts. As a result, he gains the ability to transform into a humanoid kaijuu with the strength and powers of the massive monsters menacing Japan. Dubbed "Kaijuu No. 8" by the military, Kafka resolves to use his newfound gifts for the greater good. Tied together by mutual respect, Kafka and Reno set out to join warriors like Mina at the forefront of the Defense Force.

An ordinary high school student named Bon becomes part of a team of time-traveling agents tasked with saving people's lives during historical events that happen across different eras and locations around the globe.