This is a documentary about two Scottish rappers who, after being shunned by the music industry, created elaborate personas of edgy LA hip hop stars to improve their chances at fame. Without ever having been to America they went on a rollercoaster ride of fleeting wealth and minor celebrity during two chaotic years in London.

Two Scots pass themselves off as American rappers. Sounds pretty funny, right? But what actually unfolds is closer to a grim exposé of the darker side of the music industry, and the toll the turbulent two years took on the talented twins (they’re not twins, I just needed another t word).

While the strain of sudden fame, money, parties and record deals is more than enough for anyone to handle, Gary Bain and Billy Boyd (not Pippin) had to contend with pretending to be “Silibil n Brains” 24/7 lest they be discovered and their careers ruined. What unfolds is a kind of Donnie Brasco-esque transformation as the two tell of how they sunk deeper and deeper into their personas until it became hard to tell where they ended and their characters began.

It’s mostly told by the two men themselves, with contributions from people who encountered them in the music industry, as well as home footage and cartoon reconstructions. It varies between being funny, shocking and hard to watch. Stories of meeting rap group D12, who they had in the past claim to know, or of being quizzed on their history by record executives, are nailbiting to hear about, and you really get a sense of the precariousness of the two men’s situation.

But this is as much about the music industry in general as it is about Silibil and Brains. As ordinary Scotsmen they were practically laughed out of auditions – branded the “rapping Proclaimers” – but as outrageous LA rappers they were snapped up almost immediately. Much of the strain their journey put on them and their relationships is true of many young startups in the music industry, and in some ways Gary and Billy are just lucky to have had a life to return to when things went sour.

It also asks how far it’s acceptable to go with a deception. On the one hand they were shunned by the music industry for being themselves, but on the other don’t investors have a right to know who their money is going to? And don’t most pop acts have personas anyway? How far should they have to be “themselves”? Is it possible that other acts have performed similar deceptions? Is Usher secretly Welsh? Will it turn out Snoop Dogg isn’t really a Rasta from Jamaica? The Great Hip Hop Hoax asks these questions and more.

What’s strange is that in their rapper personas the pair look like a Baron Cohen-esque parody of rappers. As Silibil and Brains they adopt obnoxious, confrontational, cocky, arrogant, and generally rather foul personalities alongside their accents which they, in large part, became. This is documented by their own filming of their youthful hi-jinx that often borders on antisocial behaviour. They look like such an outlandish stereotype it’s hard to believe anyone took them seriously. It’s not hard to believe people thought they were real, let me add, just that they were considered to be likeable or marketable.

This is a very intriguing documentary which is at times edge-of-your-seat tense, occasionally funny, often cringeworthy but never dull. It documents an extraordinary journey, but also shows the darker side of the glitz and glamour of showbiz, with two people who were fortunate enough to live to tell the tale.