Think The Devil Is a Part-Timer! meets Working!!. There's a Demon Lord on the Floor's first volume introduces a story that feels very much like a combination of those other two series' premises – Kouichi is an average guy Working!! at Humming Dining, a small family restaurant. One day he receives an employment application from a small blonde named Amon Patricia, who claims to be a demon lord from another realm. Since she was forced to flee, she now needs to survive in the human world, which means finding a way to procure food. Working!! in a restaurant seems like the most logical (or maybe just the easiest) way to do that, so here she is. Needless to say, Kouichi is skeptical of the whole thing, and he tries to pawn it off on his busty younger co-worker Mutsu. But before he knows it, things have gotten out of hand, and Amon is a new member of the team.

It should be clear from the above paragraph that hijinks are about to ensue. One of the chief features of this ecchi story is that when Amon's stomach is full, she transforms into a full-bodied older lady in appropriately skimpy clothing. (Or, if you prefer, later on she simply keeps on her waitress uniform, which stretches where she bulges.) It apparently takes too much of her power to maintain this form, so she typically cycles through it fairly quickly, but in some ways, that's part of the appeal – since we never quite know how much she's eaten at any given time (because she's really always eating), there could be an abundance of curves at any moment. For Kouichi, who is less than impressed, this sometimes means that she's riding on his back (against his will) when the transformation occurs; other times she's in the middle of waiting on a table. It adds an element of unpredictability to the story that's kind of fun, especially since Amon's otherwise a fairly typical iteration of the evil little girl trope.

Of course, she's not the only demon who shows up at Humming Dining. Almost immediately as a show of power, Amon brings forth Nonko, who is either a mushroom-girl or a girl who carries a large mushroom on her back. (I lean towards the former, but it isn't all that clear.) Later a third demon lady appears who prefers to wear her giant suit of armor at all times, which is largely played for a joke since she's too shy to be seen without it, which naturally causes lots of problems for the restaurant.

At this point, what volume one has in its favor is that Kouichi really is almost incidental to the story. He's the ultimate straight man, taking the fall for the demons, but mostly simply observing their actions and providing us with narration. Jokes that end up relying on his hard luck with these ladies do tend to land a little better than the ones based on Amon's tendency to berate the customers, but very little of the story and fanservice actually relies on using Kouichi as a typical harem lead. In some ways this makes it kind of a refreshing fanservice comedy – there are no accidental boob grabs, no awkward not-kisses, and Kouichi almost never makes a pratfall, and if he does, it isn't face-first into someone's impossible cleavage. That isn't to say that there isn't impossible cleavage or other forms of fanservice, because the art delights in its sexiness at times. But the prevailing theme of the book is demon antics in the human world and the fact that Kouichi has, without his consent, been made their keeper.

While it does feel in some ways like There's a Demon Lord on the Floor would work better as an anime than a manga, it's still a reasonably enjoyable debut. It doesn't tread much new ground, but it is having a lot of fun with its characters and story, so much so that it's almost infectious – you want to have as much fun reading it as its creators clearly had making it. While that doesn't always fully translate, if you're looking for a silly new demon girl story with pretty good fanservice and not a lot of the usual protagonist-related pranks, this is worth checking out. Even if it isn't great upon reflection, it is a good time while you're reading.