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
Aqua running around covered in toad slime with Kazuma Sato trying to calm her down to examine their loot from their last adventure. As Kazuma is going through his bag, the Mysterious button falls out of it.

In the Great Tomb of Nazarick, Albedo is discussing their strategy for world domination. Afterwards Demiurge brings up rumors of mysterious buttons appearing at random. Ainz Ooal Gown then instructs everyone to be extra cautious about it.

Kazuma stops Aqua from pressing the button, however Megumin presses it anyway because "it might change their destiny".

Ainz Ooal Gown thinks something is weird and then sits on the button that appears on his seat, teleporting them to the high school.

Once they arrive at the high school, Ainz Ooal Gown panics that he has been sent to another new world, when Aqua approaches him and asks where they are. After she realizes they are all undead, she brings tries to get Kazuma's attention. However, this just leads to the entire KonoSuba group bickering amongst themselves until Kurt von Rudersdorf sends them to class.

While in Class 2, Tanya von Degurechaff tries to figure out how she got stuck here after testing a new computation cube. As she was testing it, the mysterious button appeared and she was unable to tell which button she was supposed to press, so she pressed both of them. As Tanya was trying to decide if this was all a plot by Being X, Subaru walks in with others from Re:Zero.

Subaru is telling his group that this could be fun when he realizes the people in the room and starts to worry that may not be the case. Each group then accepts that this is going to be an interesting predicament.