The first mystery is to guess what Welles' original film was like. That makes this a real adventure -- to see an incomplete skeleton and using cinematic forensics, imagine the beautiful woman it once supported.
If you do, you will both see and experience perhaps one of the best film mysteries ever. As mysteries go, the narrative is rather ordinary: a simple diversion, one jealous husband as red herring.
What's rather miraculous is Welles' placement of the story in an artificial eye seeing a dark, dark multifaceted world. The first real noir, but even darker. It's not an obviously twisted world, unless you think about the camera. What we can see firsthand is someone creating a vocabulary that would later become common.
For all the celebration, Kane was a success because of the great drama and story. The camera's eye was shocking, but experimental. Welles would go from there to explore the mystery narrative and the self-reference of Shakespeare with this eye. Othello and MacBeth are both begun in this period, and I consider them part of a single vision with this.
The noir feel here hinges on the notion that people are not in charge of their lives, even a little -- they are manipulated by random factors in the environment. So in telling this story, Welles has to make the environment into a character. Several characters as suspects in the mystery.
Thus we have the famous lighting, blocking and angles we know (and have since seen countless times). And we have the deliberately closed sets: the park, yacht, picnic area, aquarium, dock, courtroom, Chinese theater and funhouse. I am certain that what was cut by the barbarians was lots and lots of 'external' narrative dealing not with character but with these strange environments.
My own solution to the mystery is that the funhouse did it, among the other character-environments introduced as suspects. In other words, the manipulation of Black Irish (who we know from notes and one scene typing at the union hall was an aspiring novelist) was neither: a force of human conspiracy (the park or the civilized version, the courtroom) nor of nature (the picnic or the civilized version, the aquarium).
Instead it was a matter of deliberate caprice by the gods for amusement. This is of course a self-reference to what Welles is doing: putting these people (including himself) through hell for our own caprice, a matter underscored by the Peking Opera set with Welles doped up. And of course leading to the funhouse where the environment directly tinkers with perception.
More about the self-reference: surely there is conscious comment on his relationship with his soon to be exwife. But I believe there is strong subconscious comment on his own taunting the environment in which he worked, the studio environment. Surely Welles was as much screwed with, and in much the same way, as his character. And that screwing took the form of the murder of this film, leaving the rotting corpse mentioned at the beginning of this comment.
That poor Rita comes from China ties up the whole thing, the Chinese theater, the expected rape from above, the loss of the woman. The investment in environment beyond all.

The killings in this movie isn't that bad, but for sure the movie is. It's even worse than that. It's not even worth the wear it might cause when you slide it into your DVD-player.
Not even the wear it causes on your shoulders carrying it from the DVD-store, not to speak of the money and time you spend renting and watching it. Horrible.
It's beyond understanding how anyone could say anything positive about this movie. It was just a bare masochistic tendency of mine that caused me not to stop watching, add to that the group pressure from my co-watchers.
The manuscript is awful, the directing even worse and the acting is plain despicable.
I hope you don't see this as a spoiling your fun, i'd rather see it as a fair warning: Do not waste your time watching this garbage!