"Sometimes change, like a train in the old West, gets stopped dead in its tracks. That was the story of "Who Killed the Electric Car?" The villains were the same guys who always hold things up when real progress is in the air. Pistol-waving business lobbyists fighting for their old monopolies, simpleton leaders defending the status quo, and the tendency for most of us to stay in our seats rather then board new trains.

    So it's a rare privilege to be able to tell the story of how sometimes change has too much momentum to be stopped. You can't kill an idea whose time has come.

    For this film, we wanted to do something different. Last time we followed a group of activists fighting from the outside. This time, we follow four entrepreneurs battling from the inside. Each one let us in on their journey over three years on condition that we would not release any footage until 2011.