As the Holy Empire rises to power, the neighboring lands begin to gradually fall under its control. The Empire's conquest eventually reaches the small island of Erin, home to the Gael tribe. There, a priestess named Riannon is kidnapped to be offered as a living sacrifice to the demon king Arawn, a malevolent being rumored to have caused untold destruction in the past. Riannon's brother, First Warrior Arthur, rescues her, when Arawn suddenly materializes before them as a handsome grey-haired man. Hiding his true identity and remaining enigmatic, Arawn pledges his power and leadership to the tribe's cause and joins Arthur, Riannon, and their merry band of friends—including a talented swordsman, an agile hunter, and a group of ecstatic pixies—as they fight back against the Empire, while uncovering the dark secrets of the land along the way.

The Four Cardinal Heroes are a group of ordinary men from modern-day Japan summoned to the kingdom of Melromarc to become its saviors. Melromarc is a country plagued by the Waves of Catastrophe that have repeatedly ravaged the land and brought disaster to its citizens for centuries. The four heroes are respectively bestowed a sword, spear, bow, and shield to vanquish these Waves. Naofumi Iwatani, an otaku, becomes cursed with the fate of being the Shield Hero. Armed with only a measly shield, Naofumi is belittled and ridiculed by his fellow heroes and the kingdom's people due to his weak offensive capabilities and lackluster personality. When the heroes are provided with resources and comrades to train with, Naofumi sets out with the only person willing to train alongside him, Malty Melromarc. He is soon betrayed by her, however, and becomes falsely accused of taking advantage of her. Naofumi then becomes heavily discriminated against and hated by the people of Melromarc for something he didn't do. With a raging storm of hurt and mistrust in his heart, Naofumi begins his journey of strengthening himself and his reputation. Further along however, the difficulty of being on his own sets in, so Naofumi buys a demi-human slave on the verge of death named Raphtalia to accompany him on his travels. As the Waves approach the kingdom, Naofumi and Raphtalia must fight for the survival of the kingdom and protect the people of Melromarc from their ill-fated future.

Adaptation of the Bugs 2 arc of the manga.

During the 21st century, humanity attempted to colonize Mars by sending two species which could endure the harsh environment of the planet to terraform it—algae and cockroaches. However, they did not anticipate the species' remarkable ability to adapt. Now in the 26th century, a lethal disease known as the Alien Engine Virus has arrived on Earth, and the cure is suspected to be found only on Mars. The problem is, Mars in the present is overrun by creatures known as Terraformars, incredibly powerful and intelligent humanoid cockroaches that mutated from those originally sent to the planet. The Annex I team, consisting of a hundred men and women genetically enhanced with characteristics of powerful organisms from earth, has been sent to Mars on a mission to find the cause of the Alien Engine Virus and to help cure humanity—signalling the start of the crew's fight for survival.