Come Together! Quartet

'''Come Together! Quartet' (集結！かるてっと, Shūketsu! Karutetto) is the first episode of the first season of Isekai Quartet'' and first episode of the full series overall. It aired on April 9, 2019.

Summary
Mysterious buttons appear in the worlds of Tanya the Evil, KonoSuba, Overlord, and Re:Zero. When the characters push them, they’re taken to a place they’d never imagined…

Plot
At Kazuma's Mansion, Kazuma's party back from another adventure prepare to count their spoils. Aqua cries about wanting to take a bath to wash away all the Giant Toad gunk off her. However, Kazuma stops her telling her they need to go through the items. A strange button device slips out of Kazuma's bag. In the Great Tomb of Nazarick, the Floor Guardians report to Ainz Ooal Gown on their progress in the Re-Estize Kingdom and the Baharuth Empire. Demiurge reports of strange phenomena happening within the tomb. Though he is unable to elaborate on it as he has not seen for himself, but knows it involves the appearance of a strange switch with a button on it. Ainz believes that they are under attack from an outside power which the demon agrees as it was able to breach Nazarick's defenses. Not taking any chances, Ainz orders the Guardians to send an order to report any appearances of these devices and take trap-disarming measures.

Ainz wonders if this mysterious button really is some kind of trap and comments to himself that only a complete newbie would be foolish enough to actually push it. Back with Kazuma's group, Aqua wants to push the button to see what it does, but Kazuma restrains her. Megumin seemingly agrees with Kazuma on being cautious, before pushing the button herself. The space around the group then distorts. Kazuma screams that it was a trap and the group dissolves into hysteria as they vanish.

Ainz is lost in thought when he's briefly roused by Albedo. Stating that it's nothing, he allows her to continue her assessment on the state of the Baharuth Empire. Ainz is less concerned with the New World politics and senses something amiss. Sitting down on the Throne of Kings, a button appears on the seat. The button is pressed and a spatial distortion envelops Ainz and the Floor Guardians.

The group finds themselves on the grounds of a school. Demiurge postulates that they are in another world, causing Ainz to have a headache as he had already experienced a similar situation. He is interrupted by Aqua who asks if he knows where they are. Turning to her, Aqua see that he is undead. Kazuma wonders if he has returned to his own world, but Aqua hurries back to her party pointing out that Ainz is undead and accompanied by various monsters. Darkness and Megumin are particularly thrilled at the prospect of fighting such creatures, despite Kazuma's attempts to think rationally.

Kurt von Rudersdorf then screams that its time for class to begin. Relocating to Class 2, the two groups find themselves in the midst of various characters. One, in particular, Tanya von Degurechaff wonders to herself what is going on and reflect on her past. Being that she was originally a businessman in a scientifically and ethically moral world, but later was killed and reborn as a girl in a world of magic and war. She remembers that she was forced to participate in a new computation jewel made by Adelheid von Schugel. During the test flight, she lost control and was ordered to press the button for the jewel expansion. However there two buttons when there was only supposed to be one. Unsure of which to press, she presses both and found herself at the school. Looking at her "classmates": the adventurers and monsters, she wonders if this is the result of Being X.

A new group comes in through the classroom, revealing Natsuki Subaru and his party of friends. Initially excited to be back at school, it quickly changes to fright upon seeing the occupants of the classrooms. Ainz voices his annoyance of the new addition to the already confusing situation. Albedo suggests to Ainz that they slaughter everyone, but Ainz tells her that they need information first and she unhesitatingly obeys. Everyone is confused at this situation but nonetheless, several members are hopeful for the future.

In the principal's office, Kurt von Rudersdorf, accompanied by Hans von Zettour states that 'their' school life is about to begin.

Trivia

 * By Albedo's dialogue, the events of this episode take place after the Sorcerer Kingdom's annexation of the Baharuth Empire.