Here's Wiki's brief talk of it: The series begins with the sudden arrival of alien entities known as the Borant Corporation, who restructure subterranean Earth into a multi-level dungeon for their game show. Humans and various Earth creatures are given the option to become "contestants" in a live-streamed, gladiatorial game show, Dungeon Crawler World, or otherwise stay on the surface of the Earth which has just been stripped of all modern amenities.
Carl, having just been cheated on by his girlfriend and left with her cat, Princess Donut, decides to enter the dungeon and finds himself thrust into a deadly competition with uncountable viewers across the galaxy watching their every move. Upon entering the dungeon, all "crawlers" are given a virtual HUD along with an assignment of stats representing their physical and mental abilities which can later be increased and various other enhancements. Early in the first novel Princess Donut becomes able to communicate with humans due to a special consumable item known as a Pet Biscuit that Carl received for bringing the first cat into the dungeon. The biscuit also enhances Princess Donut's base stats, causing the dungeon to view her as the leader of her and Carl's two person group, which she dubs the "Royal Court of Princess Donut".