Alex Gibney once set out to make a film documenting Lance Armstrong's sporting accolades.

What he wound up untangling was a web of lies centred around the now disgraced cyclist's fall from glory, including how he was at the centre of a doping scandal that rocked the cycling world and left his image tarnished.

In his documentary The Armstrong Lie, which is being shown at the Toronto International Film Festival this year, the Academy Award-winning filmmaker goes behind the scenes with the former cycling star.

Click the above video to see how it happened.