Upon hearing news that his mother was on verge of death, Haru Kaidou—the eldest son of the family—flies all the way to Canada. The moment he arrives, he learns that not only did his mother fool him, but he is also supposed to take care of his adoptive brother, Ren Kaidou, an antisocial kid who feels more comfortable around dogs than people. Due to his new brother's distrustful nature, Haru initially has a hard time reaching out to Ren but their relationship eventually grows. He makes a promise to Ren: they will live together in Japan after Haru graduates from high school. However, due to an unfortunate accident, Haru loses all memories of the summer they spent together, including the promise he made. Five years later, expecting Haru to keep his promise, Ren arrives in Tokyo; but to Haru, Ren is just a random boy claiming to be his brother.

With another Wave happening in a week, Naofumi Iwatani and his party have no time to waste. However, when bat familiars raid Lurolona Village and the Wave countdown comes to a halt, the Four Cardinal Heroes reconvene with the queen, Mirelia Q Melromarc, for a quick briefing. The queen presumes that the odd occurrences are linked to the Spirit Tortoise—a threatening creature that has awakened from its slumber, back to cause havoc once again. A plan to put the Spirit Tortoise to rest is devised—but out of the four men, only the cursed Shield Hero agrees to help.

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.

On his way to a convenience store, Kazuya Shiina tries to defend two young girls from an armed assailant, only to end up getting stabbed himself and dying on the spot. This good deed allows him to be reborn with all his memories in another world as Cain von Silford, the third son of a margrave. When Cain turns five, a baptismal ceremony in the church grants him an audience to the seven gods who preside over the world. Not only do the gods fill him in on the circumstances of his reincarnation, but they also give Cain their respective divine protections—allowing him to use cheat-like abilities in swordsmanship, magic, and many other aspects. With these blessings, Cain sets out to live a fulfilling life that he deems a dream come true. However, it soon becomes clear that the gods have more plans for him than just a haphazard bestowal of overwhelming power.