Doctor Emmett Lathrop "Doc" Brown is a fictional character, one of the lead characters in the Back to the Future motion picture trilogy, played by actor Christopher Lloyd.
On November 5, 1955 Doc came up with the idea of the flux capacitor after slipping off his toilet while standing on it to hang a clock and bumping his head. The idea came to him in a vision he had after being knocked out.
In order to travel through time it requires 1.21 gigawatts (GW) of electricity originally supplied by some kind of a plutonium powered nuclear reactor. Also for time travel, the De Lorean needs to be travelling at 88 mph (141.6 km/h), which is achieved by its gasoline powered internal combustion engine. During its first two trips (one minute forward in time, and back in time to 1955) the combination was used without incident. However, during its next trip (back to 1985), plutonium was not available and a lightning rod was connected directly to the flux capacitor and was used while the vehicle sustained 88 mph. Plutonium was used once again for a trip forward in time at least 30 years, and at some point thereafter the plutonium reactor was replaced by a "Mr. Fusion" home energy generator from the future that was somehow fueled by garbage. The De Lorean once again came back to 1985 and proceeded to travel to 2015 where it was stolen and taken back to 1955 and returned back to 2015 without the Doc's knowledge. When they returned in 1985, they found it was a different present, so they traveled back to 1955 to fix the timeline. The De Lorean was again struck by lightning in the year 1955, this time by accident. The lightning activated the flux capacitor and sent the De Lorean back to 1885. It is unknown exactly how this happened, as the De Lorean was hovering in midair and not traveling at 88 mph, but it is likely some combination of the severe electrical storm mixed with the malfunctioning time circuits. The De Lorean's time circuit control microchip was irrepairably damaged so it was hidden in the Delgado mine for 70 years. It was recovered from the mine in 1955 and repaired with "1955 counterparts", thus restoring it to perfect working order. Since both gasoline and garbage were available, the next trip back to 1885 was performed under the car's own power. Due to lack of gasoline and a broken fuel line, the De Lorean's final trip from 1885 to 1985 was partially powered by a steam locomotive pushing the vehicle up to 88 mph while using Mr. Fusion to generate the 1.21 Gigawatts required to activate the Flux Capacitor and break the Time barrier. At the very end of the third movie, Doc returns to 1985 in a time machine made from a train. This uses a flux capacitor that Doc was able to power using steam.
Trivia: Doc's first name Emmett comes from the word "time", spelled backwards and pronounced as syllables (em-it). His middle name Lathrop spelled backwards gives (almost) "portal". Whence "time portal".