The book begins with the death of 99.99% of humanity. Basically, everyone who was indoors during the event is killed instantly, including cars and planes. Aliens have come to claim the Earth and offer the surviving humans a choice: they can either participate in a massive live action fantasy RPG with life and death stakes or die in the ruins of their dead world. Carl, who had been getting his ex-girlfriend's cat in his boxers, chooses the former. Ironically, his cat is added as a player and is uplifted to superhuman intelligence by participating in the game as well.
The chief draw of the book is the strange relationship between Carl and his cat partner, Donut. Donut is a show cat that has been spoiled and retains memories of things like Gilmore Girls as well as how Carl's ex-girlfriend used to cheat on him constantly. It's adorable and the fact Donut's enhanced intelligence puts her above Carl's own plus the fact she's pathologically addicted to fame is the source of much of the book's humor.
Indeed, part of what makes the books so entertaining is its commitment to realizing how utterly ridiculous the situation is yet playing it straight. Carl and the remaining survivors of humanity are trapped in what amounts to an utterly insane game show run by incredibly immature [expletives]. All the magical objects and achievements have sarcastic mocking descriptions and the people running the show are bored xenophobic sociopaths or corporate shills (or both).
This is a very funny book and yet it maintains just enough of seriousness about how horrible Carl's circumstances (let alone humanity's) are to give actual stakes to what is going on. Humanity's extinction is very likely and there's probably nothing Carl can do to prevent it, but he can certainly try to gum up the works as much as possible. He can also attempt to save Princess Donut, who is the best kitty in the universe.
The rules and magic system are functional but mostly immaterial. Carl is always the underdog in any given situation because he's always taking on more than he can chew. He also has to deal with player-killing fellow Crawlers who realize the best way to survive is probably to do less killing of NPCs (who are living thinking constructs themselves) and more their fellow confused humans. This is in addition to the massive number of mutated humans, monsters, and other weirdness Carl encounters on his journey through the first couple of levels.
In conclusion, this is a very fun book and full of dark humor that makes it a joy to read. After all, what sort of book can have its hero accidentally blow up a bunch of goblin children, get an achievement for it (complete with lootbox), and then have the description of it call him out for being such a complete monster? Yes, it is that sort of book.