From illegal weapons to the underground sex trade, nothing is off limits and everything is for sale in illegal, underground markets. It's an industry that is brutal, exploitative, and corrupt, where what you buy isn't always what it seems. Despite efforts to crack down on these underground trades, more than $625 billion makes its way into the US economy each year. Unprecedented inside access sheds light on the inner workings of the last bastion of 'free' enterprise, charting a global path to the underworld economy. Workers, suppliers, sellers, customers, and the law enforcement trying to keep it in check, share their experience of a hidden world that touches us all.