Directed by Welles from his own novel, this is the only movie he ever made that was not from somebody else's story. It is not strong, and the editing (not by Welles, of course) was terrible. You would probably be better off reading the book (even though Welles later claimed that not only hadn't he actually written it, and had never even read it).
However, its faults aside, this movie is important for one reason, which is the story of the scorpion. Mr. Arkadin, a rich and powerful man (played by Welles) tells this story at a party. I paraphrase:
A scorpion wishes to cross a stream, but scorpions cannot swim. So, he asks a frog to carry him across on his back. The frog refuses, saying that the scorpion will sting him. The scorpion points out that if he stings the frog while they are going across the river, they will both drown.
The frog sees the logic of this argument, and starts across, bearing the scorpion on his back. Halfway across, he feel the scorpion's sting. "Is this logic?" he cries as they sink beneath the water.
"No, it is not," replies the scorpion, "but I can't help myself. It is my character."
This story is very important for understanding the view of human nature which Welles brought to all of his work. In most of his films, there are scorpions and frogs, and the scorpions always keep to their true nature even when it means their own destruction.
It is even important for understanding how he conducted his career (see my review of Ambersons for his comments on his films). He had to make his films his way, to be true to his character.
(The story is already familiar to anybody who has seen The Crying Game, of course.)
Later: Welles' original version of the movie is available on the Criterion Collection DVD, which includes three versions of the movie in total. It is much improved over the versions which had been generally available until now. It does not alter the meaning of the film (unlike Touch of Evil), but it does make a much better story, focusing the attention on the most interesting character (and the best performance).