Official story from the Re:Zero mobile game: Re:Zero INFINITY Machine translated but I edited the wonky grammar and confusing scenarios for a more digestible format similar to a real translation.

After (Y/N) (L/N)'s Crazy Adventures in his Reincarnation and met Kazuma and the others that he had met so far. So right now, one day they see a big red button on the table of the mansion. Kazuma was cautious, but Megumin had literally pushed the button, as (Y/N) and the others were transported back into the modern world. However, they met with other people that were monsters, generals, and others. (Y/N) had one thought to himself... "Is this a Crossover?"