During an experiment, Hatsume managed to contact an alternate universe version of herself and with the help of that version of herself she was able to a teleporter to that universe after a few weeks.

Class 1-A, their teacher Aizawa, and several pro heroes, such as Endeavor and Best Jeanist, volunteered to test out the teleporter, but when it activated they were sent to a destroyed version of Japan

Unknown to everyone, even Hatsume, was that the alternate version of herself was from a universe where a zombie outbreak destroyed everything and that it was a zombie version of herself, who tricked Hatsume into making the portal so that the zombies of this universe could get more people to eat.

In this universe an outbreak of a zombie virus happened that turned people into the undead while they still retained sentience and the ability to speak but experienced such a great hunger for human flesh that they were unable to stop themselves from killing and eating the living

Guest was a member of class 1-A in this universe and was hiding on the rooftop of an abandoned building searching for a safe place for the survivors of the outbreak to use as a new base when they saw class 1-A teleport to this dimension. Guest was surprised to see an alternate version of themselves and their class from a world that hadn't experienced the apocalypse, especially since in this universe the rest of class 1-A had been some of the first victims of the outbreak