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…