Agents of S.H.I.E.L.D. introduced Inhumans as an MCU substitute for mutants, since everything "mutant" is licensed to Fox. Ever since that introduction, however, readers of the comics have wondered what the MCU would do with the original, proper Inhumans: Black Bolt, Medusa, and the rest of the Inhuman Royal Family who rule over the secret Inhuman city-state of Attilan, on the Moon. Well, this series answers that question. Badly.
Inhumans are a result of genetic experimentation by the Kree (i.e., the blue-skinned humanoid aliens in Guardians of the Galaxy) on prehistoric humans. When the Kree abandoned Earth for various reasons, the remaining Inhumans formed their own society separate from the more primitive humans. Then, as humans continued to become civilized and populate the earth, the Inhumans decided to remove themselves to the Moon. The genetic potential to become Inhuman still existed in the human population, which is how Inhumans were introduced in Agents of S.H.I.E.L.D., but those Inhumans don't know anything about the Moon-based Inhumans.
NOTE: Yes, the Inhumans and Attilan are the basis for the legend of Atlantis in the Marvel Universe. Except that Namor the Sub-Mariner also rules a city called Atlantis on Earth… Yeah, it's complicated, as comic book continuity often is.
Fast forward thousands of years to today. The Inhumans live in a tiny city-state hidden in a crater on the Moon. They call it a "kingdom" but it's really no more than a small town, and it's not allowed to grow for fear of being discovered by Earth. (What do they have to fear? Well, they may be superpowered but they're outnumbered and outgunned.) Space and resources are severely limited, so over time they've been forced into a rigid caste system that is ruled over by the Inhuman Royal Family. As you can imagine, there's plenty of social unrest, and it's being stirred up even more by the king's "evil" younger brother Maximus (Iwan Rheon).
The premise of this series, as layed out in the first two episodes, is that Maximus just can't take it anymore and he finally launches his long-planned coup against the rest of the royal family. They are forced to flee to Hawai'i by way of a giant teleporting bulldog named Lockjaw, but since Lockjaw extracts each family member one at a time during the coup, and since his teleport is less accurate over long distances, they all end up scattered and lost around the island of Oahu. (Guess who inherited the production tax credits from Lost!) From there, all of the family members start their own, separate storylines, meeting and getting involved with assorted residents of the island.
Agents of S.H.I.E.L.D. kinda-sorta works because it has three things: a known and likable main character in the form of Agent Coulson; a somewhat more familiar premise for a television series (yay, super-spies!); and enough of that Whedon-esque sense of humor to not take itself too seriously. These things allow it to overcome its low budget and mediocre writing.
Inhumans has the same low (possibly lower) budget and mediocre writing but none of the other things. It's just laughably bad. It's all about superhumans that most viewers don't know, and it takes itself way too seriously. Its biggest mistake, I think, is that it makes it very difficult to empathize with the Inhuman Royal Family. They're all portrayed as entitled assholes who take the Attilan caste system for granted because they sit at the top of it. It's obvious that the long-term story arc of the series is to have them broken and humiliated by the coup, so that they can subsequently learn and grow during their adventures among the lowly humans on Earth, but I don't care enough about any of them at the beginning in order to follow that arc.
In contrast, Maximus's coup seems downright justified: he promises a free and open society without caste, and he wants to at least consider repatriating the Attilan population to Earth since more Inhumans are appearing there (due to events in Agents of S.H.I.E.L.D.). Furthermore, he doesn't do anything conspicuously evil except murder the head of the Inhuman Genetic Council that enforces the caste system. It was an interesting choice to cast Iwan Rheon as Maximus: the producers obviously wanted to get some of that Game of Thrones cachet, and at first glance it might appear to be type-casting, but Maximus is actually more calculating and less sadistic than Ramsay Bolton was. Rheon could do a lot with the role if the writing were better. It's not, however, and tyrants gotta tyrant, so… <shrug>
I haven't decided yet whether I'm going to waste any more time on this series. I watched as much as I did only because I think the episodes will start expiring off of Hulu.