A summary of season one of Tensei shitara Slime Datta Ken with some extra content leading into season two.

Twelve-year-old Linn Youki loyally supported his father when the world laughed at the scientist for saying that Earth was about to be invaded from outer space. But Linn did not expect to become personally involved--until he met Takoro, the strange young "deputy sheriff" from another world who was on the trail of notorious space criminals. There were only four Kurodako Brothers, but they plotted to use their alien science and natural shape-changing powers to become the secret masters of Earth. With Takoro's help, Linn became the mighty super-hero, Muteking, to foil their schemes. There is plenty of action and laughs in this light-hearted comedy-drama about a roller-skating costumed hero, his friendly but bumbling alien partner, a gang of klutzy would-be world conquerors, a beautiful police chief, an obsessed scientist, a sarcastic dog, an officious flying saucer, and others. This unusual but appealing cast of characters will charm viewers' hearts while it amuses them.

Long ago, the king of Orvelia, Kyle, defeated the demon king Angmund and brought peace to the world. However, one hundred years later, demons have been sighted in the forests, threatening humanity once more. A scouting expedition is sent to verify the claim but only one member returns. Meanwhile, Riheet, the leader of a dark elf mercenary group, plots to take over Orvelia to exact revenge against the humans who forsook their race a century ago. The knight apprentice Kasel, accompanied by the priestess Frey, sets out to rescue Clause, an old friend who went missing in the tragic mission. However, along the way, Kasel discovers that he is the son of the revered King Kyle and the only one who can wield the Holy Sword Aea—the same sword that slew the demon king. To fulfill this destiny, the young knight must embark on a perilous quest, unseal the sword, and end the fear instilled by demons. As Kasel's journey to bring hope to humanity and Riheet's vow for vengeance intertwine, what fate could possibly await them?

Atsuko "Akko-chan" Kagami (known variously as "Stilly," "Caroline," or "Julie" in Western versions of the anime) is an energetic elementary school girl who has an affinity for mirrors. One day, her favorite mirror which was given to Akko by her mother (or in some versions, by her father, as a present from India) is broken, and she prefers to bury it in her yard rather than throw it to the trash can. In her dreams, she is contacted by a spirit (or in some cases the Queen of the Mirror Kingdom) who is moved that the little girl would treat the mirror so respectfully and not simply throw it away. Akko-chan is then given the gift of a magical mirror and taught an enchantment that will allow her to transform into anything she wishes.