"I am a charlatan."
Welles' films are mostly quite somber in tone. There is little humor, except in Falstaff, but Welles had another side that came out in his magic act, and in interviews. He was a trickster, a ham, a gleeful illusionist. This side dominates this film, which is more or less a documentary (a film essay he called it, which he hoped would catch on as a new genre of filmmaking).
He set out to make a documentary about Elmyr de Houry, one of the greatest art forgers in history, whose paintings hang in museums throughout the world. Actually, Welles saw a documentary about Elmyr on French television and bought up all the footage, including outtakes, in order to expand it into a feature film. But the unpredictable element was that the expert on Elmyr in the documentary was Clifford Irving, and in the middle of the making of Welles' movie it was revealed that Irving was quite a forger and illusionist himself (he wrote a fake Howard Hughes autobiography which he sold for a tidy sum, and then the whole scam was discovered).
At that point, the film became much more of a meditation on illusion, trickery, and authorship in general, especially since Welles claimed that he had originally intended to make Citizen Kane about Howard Hughes.
But, as he said, if you told the story of Hughes' life in a film, nobody would believe it.
He also performs a bit of trickery on the audience, which is revealed at the end of the film. I won't give it away.