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.