You were sitting peacefully in your room, but just as you were about to lie down and learn some new spells, your sister Megumin ran into the room Megumin: We're going training, now! Without waiting for you to answer she pulls you behind her as she runs into the forest where she always uses her single explosion, firing it at some abandoned castle deep in the forest she had found, that withstands her explosions somehow after years, calling the process "training" and you get to carry her home, as always.