I'm way too late to the game seeing this movie and reading the story upon which it's based, '--All You Zombies--' by Robert Heinlein; so, I knocked out reading the story and watching the movie in the same day.
I know the movie is almost ten years old and the story is over sixty; but, just to be safe, I'll warn about spoilers below.
What I'll say broadly is that they're both great works in their own right, the adaptation is actually very faithful to the story, and it's also not surprising that it took half a century for an adaptation to emerge.
Heinlein's story and the movie both deal with the same time-travel paradox regarding the same character(s); but, Heinlein's story was a great work of literature without being particularly cinematic. '--All You Zombies--" is one of the most worthwhile short reads in the science fiction. It's one of the few time-travel that I can think of without glaring holes in the logic and it deals with the paradox on a very personal scale.
The movie accomplishes the same thing in its own way and uses most of the source material verbatim. It's really a clinic in how to adapt a great work of literary fiction. The filmmakers managed to keep everything about the spirit and the personal drama of the original story while adding visual elements and extra story components that both helped smooth the transition from the page to the screen and make the movie a full movie without making it feel like they were throwing elements in there for time.
Needless to say, I recommend both. The story is only 19 pages and the movie is only 94 minutes; so, it's not hard to find time for them eventually.
--Okay, spoilers begin--
You really can't talk about the story of the story and the film without dealing with the fact that the character(s) is intersex and all of the main characters are actually the same person.
In the book, the three characters that we meet are the Bartender, the Unmarried Mother (who we're introduced to as a man), and Jane.
These are all the same person at different points in their lives who are interacting with each other via time-travel. It's kinda the opposite of the whole "going back in time and killing your own grandfather" thing. All of the characters are their own mother, father, and child at the same time.
The movie creates a bit more of a motivation for this in creating one person, one agent with no connection to time. One person is their own father, mother, and - ultimately - killer.
This is hard sci-fi done well across the board. The stories follow their own rules closely. In both cases, it's not just a cool sci-fi gimmick. There are real human stakes and moral questions.
