As the battery begins to wind down, a small gasoline engine begins generating the electricity needed to keep driving.
The really nice thing about this plan, in GM's view, is that the gasoline engine could easily be swapped out with any other kind of generator. If hydrogen fuel cells finally start to catch on, no big reengineering project will be needed. Just drop in a fuel cell instead of an engine and a compressed hydrogen tank instead of a gas tank.
By the time the Volt comes out in late 2010 other carmakers, as well as GM's own Saturn division, will offer plug-ins - both hybrid and all-electric - for sale, as well. A century after it was born, the plugged-in car may officially be back.