Overlord is a “trapped in a game” story, similar to Tron or Log Horizon. The rules here are a little different from some other “stuck in virtual reality” stories. It is just one guy (username Momonga) who is trapped instead of the usual group of people. Despite not having other players trapped with his, the NPCs controlled by his guild have all come to life. They are now real people, but their personalities stay just like in the game. Saying they view Momonga as the boss is an understatement; he is more like a god to them.
This world they are trapped in is a bit different from similar stories as well. The video game Momonga was playing, Yggdrasil, is not where they are. They have been transported to an entirely new world; they are in unknown territory. By and large Momonga and his Guardians (essentially generals who defended his guild base from invaders in Yggdrasil) seem to be extremely powerful in this new world, but for all they know something just around the corner could be tough enough tOverlord, o crush them like ants. Interestingly, the book does not mention how or why Momonga got stuck in this new world. It just…happens. Presumably, that is a point that will be covered in the later books.
As characters, Momonga and the Guardians seem more in line with villains than protagonists. You have a lich (Momonga), demon man, a pair of dark elves, giant bug-man, vampire girl, demon lady, and a butler who probably is not human either. Beyond Momonga, there is not too much character development in this book. At the beginning of the book it is shown that each of the NPC characters had backstories in the game, so we will probably learn more about them later on in the series.
Usually the goal in these “trapped” stories is to get out and go home. Momonga has absolutely no interest in doing that. On the contrary, he seems happy with this turn of events. He has gone from being a video game obsessed office worker to a supreme overlord with an army overnight. Momonga is, if anything, the bad guy. He has become an (evil?) undead with a legion of monsters ready to serve him. By no means is he a nice person in this story, but he is not unnecessarily killing or torturing people either. But he would do both of those things if it furthered his goals. A lot of things in this story seem to be on the flipside of what you would usually expect and that is what makes it so interesting.