In the film "Back to the Future," plutonium makes Doc Brown’s time machine work.
Doc builds the machine into a modified DeLorean DMC-12 and connects it to a device called the flux capacitor. The plutonium powers a nuclear reactor in the car, producing the enormous energy the machine needs to achieve time travel. The film describes the required output as 1.21 gigawatts.
The plutonium’s backstory is part of the movie’s opening crisis: Doc obtained it from Libyan terrorists after promising to build them a nuclear weapon, then gave them a fake bomb casing filled with pinball-machine parts. When Marty accidentally travels from 1985 to 1955, the DeLorean no longer has enough plutonium for another trip.
A common mix-up is lightning. A lightning strike does power the DeLorean when Marty returns to 1985, but it is an emergency substitute rather than the machine’s usual power source. In the later films, the time machine’s power arrangements change, including the use of household garbage in the DeLorean’s Mr. Fusion generator.