Once upon a time

Sans, Isa, and Erza were taking a simple stroll through the woods, looking for adventure, when they stumble upon what looked like a trashed lab. While exploring the creepy place, Sans presses a button at an attempt to annoy Isa, thinking the lab was completely broken down. That button opened a rip between dimensions, a portal, taking the three girls to a place where they go on adventures, make friends, and discover their true identities.