Robert Leroy Johnson is a very impactful blues singer steeped in much mystery and myth. What little we know is that he became a master at the guitar and recorded 29 songs via 2 recording session over the span of 7 months, then promptly died at 27, starting the myth of the "27 club". Beyond that, much is speculation.
If you're hoping to get more info about Johnson, this is not the documentary for you. Calling it a documentary is really a misnomer given how much speculation and free liberty it takes. For instance, a music historian visits an old run down cabin in Mississippi and says "we can say that this is the house Robert Johnson grew up... probably." Come on. It's one thing to say he came from a very poor black family in the early 1900s and this most likely his living conditions, it's another to just flat out lie and say this is his house.
The "documentary" pulls this stunt again and again, from showing a random woman who claimed to have dated him after a reporter drove around offering people money to anyone who could tell him a story about Robert Johnson... to wildly implying he was poisoned when his death certificate does not list a cause of death. It's a popular myth, but one that has never been proven. Most likely he had a congenital disease given his abnormally large fingers and early death. It also didn't help that the man was notoriously known to regularly drink an entire bottle of whiskey a night, minimum.
Unfortunately little facts are revealed in this "documentary," which is a shame because Robert Leroy Johnson is considered one of the most influential musicians in modern history, with many considering him the true first "rock star". I think it would've been better if they took the time to analyze what made his sound so unique for the time, how he went about recording his music, and what impact he had on the industry decades later. The end of the documentary does a quick 2-3 minute summary of various musicians from the 50s-70s taking inspiration from Johnson, but this could've been easily flushed out to really convey Johnson's significance for those who may have never heard of him.