After falling asleep while playing an online game, a man wakes up and finds himself transported to that game's world as his in-game character, Arc. He wastes no time adjusting to his new environment, but soon realizes that he is using the skeleton avatar he chose when creating his character, which forces him to hide his visage to avoid unwanted attention. Arc goes to the nearby castle town of Luvierte, hoping to become an adventurer and take on quests to earn money. With his overpowered abilities, Arc embarks on a fantastical journey—exploring diverse territories, looting various monsters, and helping people get out of sticky situations. However, his seemingly innocuous actions may soon involve him in a brewing large-scale conflict that will forever alter the fate of the realm.

Seiken Tsukai no World Break takes place at Akane Private Academy where students who possess memories of their previous lives are being trained to use Ancestral Arts so that they can serve as defenders against monsters, called Metaphysicals, who randomly attack. Known as saviors, the students are broken up into two categories: the kurogane who are able to use their prana to summon offensive weapons and the kuroma who are able to use magic. The story begins six months prior to the major climax of the series during the opening ceremonies on the first day of the school year. After the ceremony is over, the main character, Moroha Haimura, meets a girl named Satsuki Ranjou who reveals that she was Moroha's little sister in a past life where Moroha was a heroic prince capable of slaying entire armies with his sword skills. Soon afterwards he meets another girl, Shizuno Urushibara, who eventually reveals that she also knew Moroha in an entirely different past life where he was a dark lord capable of using destructive magic but saved her from a life of slavery. Can those whose minds live in both the present and the past truly reach a bright future? Delve into the complex world of Seiken Tsukai no World Break to find out!

Seven Japanese high school students enjoy international renown for their remarkable talents. One day, these friends survive a plane crash only to find themselves in the medieval fantasy world of Freyjagard, where two human races live side by side in a feudal society: the byuma, who have animal features and formidable strength, and the hyuma, who have a small chance of magical aptitude. After being rescued by the byuma Winona and her adopted elven daughter Lyrule, the group pledges to use their advanced skills and knowledge to pay back the people of Elm Village for their hospitality and find a way to return back home. Tsukasa Mikogami, the prime minister of Japan, acts as the leader of these young geniuses and organizes their efforts to intervene in Freyjagard and gather the information and resources necessary for achieving their goals. Believing that there is a connection between their current situation and an ancient legend about seven heroes from another world who defeated an evil dragon, Tsukasa directs the others to learn about the culture around them and search for any clues leading them back to Earth. But he also gives another instruction: to take it nice and easy, lest they ruin this world by giving it their all.

Though he has successfully repelled the latest Wave of Catastrophe, Naofumi Iwatani—the Shield Hero—has no time to rest. Naofumi is spurred back into action when Queen Mirelia Q Melromarc tells him that the three other Cardinal Heroes have disappeared without a trace, and she tasks him with finding them. With no clues regarding their whereabouts, Naofumi tackles a more pressing issue: the worsening slave trade of demi-humans. Determined to right this injustice, he and his friends find their way to Zeltoble, the country of mercenaries, where the illegal sale of demi-humans has become a lucrative business. To gather money to free the slaves, Naofumi and his comrades disguise themselves and take part in underground coliseum brawls, purposely throwing matches to manipulate the odds and eventually secure an enormous payout. However, with the threat of another Wave of Catastrophe looming on the horizon, the Shield Hero must stick to his mission of finding the missing heroes if he wants to protect those he loves.