"Legion" is the story of David Haller, who's in an psychiatric hospital being treated for schizophrenia when the series starts. Problem is, it rapidly becomes apparent in the first episode that - as messed up and unreliable as he is as our first person witness - he's also a powerful mutant and people are interested in him. We spend a lot of time replaying his childhood and teen years in the first three or four episodes - and only in later episodes do we become fairly clear on which of his friends are real.
Some of the musical choices are astonishingly good: the opening song of the first episode was the Who's "Happy Jack," which plays over a montage of David's childhood - "the kids couldn't hurt Jack, they tried and tried and tried" - David is a powerful mutant unbothered by the abuse of other children, and the song worked very well with the visuals. The music choices were eclectic and occasionally brilliant.
Much of what happens takes place in "The Astral Realm," a place Dr. Strange likes to visit a lot. It's a construct of the mind, and anything goes there. Problem is, some people don't know they've been brought there (if it appears to be the real world) and that this is now a battle of the minds ...
As a whole the series is deeply weird but mostly fairly well written which keeps it interesting.
I find it particularly ironic after such a non-chronological and surreal series that I was offended by what amounted to an alien abduction at the end - I didn't think it fit with the tone of the series. (Yes, I know they'll have some other explanation next season.) The other huge problem was that the ending was a cliffhanger: I've argued before that a cliffhanger ending shows that the writer has no faith in themselves or their viewers. The writer thinks they haven't written something good enough that people would come back without a threat to the lives of major characters. And it's one of the best ways to drive me away from a series (books or TV) - which is unfortunate, as I was kind of enjoying this one.