Doctor Emmett Brown (‘Doc’) created his time machine in October 1985 after being inspired by an idea 30 years earlier. Some of his first temporal experiments were trips to 1955 and the far-off future at the time, the year 2015.
The key to time travel is the flux capacitor, designed by Doc in 1955, but unable to be tested for another 30 years in 1985 when he used radioactive plutonium to fuel the time-travel adventures.
When Marty McFly travel back to 1955 with Doc’s 1985 time machine, the Doc was unable to fuel the DeLorean due to radioactive plutonium not being readily available.
But with Marty being from 1985, he had secret knowledge from the future, more accurately, he knew of a lightning strike on the clock tower that was coming up in a few days.
Doc modified the time machine in 1955 to take the 1.21 gigawatts from a bolt of lightning and channel it into the Flux Capacitor through Marty driving the time machine and it’s extended rod into a wire that ran from the clock tower so the moment lightning struck, the energy would be channeled to power the DeLorean.
On a trip to 2015, Doc had a made some adjustments and instead of the time-traveling Delorean having to drive up the road 88mph in order to travel, he used thrusters to fly the Delorean and time travel in the air.