While it's pretty much agreed upon by historians that King Arthur never actually existed, many, many average folk think he did. Because of this mistaken assumption, the folks who made "Mystery Files" capitalized on this. In other words, they try to convince the reader that all the stories (written 500-1000 years AFTER the fictional character supposedly died) are actually based on someone like the character--that there was a historical basis for the myths. As a retired history teacher, I want to clearly say 'the guy is fake--and he's just a character in a lot of stories and folk tales...PERIOD!!!'. But, for almost 30 minutes, the folks who made this show work very hard to convince you he MIGHT have...I say MIGHT. But they present no hard proof and I felt that the show was pretty limp as a result.
By the way, as an analysis of the literature of Arthur, the show isn't all that great either--picking and choosing which Arthurian stories to discuss and ignoring others. Not very systematic nor exhaustive in their approach.