Founder's Day, the day celebrating the founding of Hill Valley, is approaching and Doc is in charge of the food booth. He's invented the ELB "Hot-Diggity-Dogger" which will make the perfect hot dog. Sheriff Taylor phones Doc and warns him not to mess up with any inventions and ruin Founder's Day.
Afterwards, Doc tries cleaning up his lab, but the cleaning machine explodes sending soap suds everywhere. Clara takes the boys to the Tractor Pull Contest, telling Doc to find a new lab. Doc goes to the McFly house and Marty lets him use the garage. Doc needs power, so connects to the pylon outside the McFly house. Unfortunately this causes a power overload, and as such all of Hill Valley is left in the dark - literally!
The people of Hill Valley - including Biff - are angry at Doc and want him and his family out of town. Doc decides to make amends and goes to the Hardware Hut to buy some tools - but the Sheriff catches him and drives him to the city limits, saying that Doc is no longer welcome in Hill Valley. If he comes back into the city, he will be arrested.
Doc sneaks back to Lyon Estates and with the help of Marty, disguises himself first as a cowboy and then as a geisha, to get the tools. Doc invents a generator that will power Hill Valley for free! The machine is a success and the power is returned.
Unfortunately, the invention is a little bit too successful - all the lights are on permanently! As the angry people can't sleep, Biff gets a mob together to run the Browns out of town. Doc offers to fix the machine - so does Biff. The people pick Biff, but Biff spills soda over it! Doc has an idea and manages to get inside the machine and pull the plug, causing the lights to go off again.
Next day, Founder's Day, sees the town once again without any power. The people are sad about this: No bumper cars, rock concerts, or anything. Then Clara appears with an old man who reminds them that back when the city was founded, they didn't have electricity. The people decide to celebrate Founder's Day just like the founders did, and Doc discovers the old man is actually the Old Pioneer, whose statue is in Courthouse Square. As the townspeople celebrate Founder's Day, Clara takes the Old Pioneer back to the 1850s in the DeLorean.