The flux capacitor was the core component of Dr. Emmett Brown's time traveling DeLorean time machine and the following Jules Verne Train. Doc stated that the flux capacitor "is what makes time travel possible."

On November 5, 1955, Dr. Emmett Brown came up with the idea of the flux capacitor after slipping and bumping his head while standing on his toilet to hang a clock. The idea came to him in a vision he had after being knocked out. He drew up a schematic diagram of an inverted Y-shape with wires and stated "flux compression". He also performed some mild calculations on the paper.

Since coming up with the idea, Doc spent many years and most of his family fortune trying to get it to work. By 1962, he had invented the temporal field capacitor. It was a prototype time machine that was able to send objects through time, but only during the lifespan of the device. It could only send objects forward through time, as sending objects even a few minutes into the past would cause a buildup of flux energy. If an object was sent any further back than that, the arrival of the object would overheat the capacitor and start an electrical fire. The problem with the build-up of flux energy was solved by the flux capacitor, allowing time travel to the future, as well as the past.

The flux capacitor consisted of a box with three small, flashing incandescent lamps arranged as a "Y", located above and behind the passenger seat of the time machine. As the car neared 88 miles per hour, the light of the flux capacitor pulsed faster until it had a steady stream of light, which one was not supposed to look at as indicated by the Dymo warning label SHIELD EYES FROM LIGHT placed across the glass panel. The stainless steel body of the DeLorean also had a beneficial effect on the "flux dispersal" as the capacitor activated, although Doc was interrupted by the arrival of the Libyans before he could finish explaining it to Marty McFly fully. Accessing the flux capacitor safely required disconnection of the capacitor drive, as the Dymo warning label at the top of the unit — DISCONNECT CAPACITOR DRIVE BEFORE OPENING — pointed out.

The coils that can be seen across the front and along the rear sides can be referenced as the temporal demodulation coils (as used in the original blueprints of the vehicle). These play a key part to open a hole in the time barrier.

The flux capacitor can be seen lit up behind Marty just before temporal displacement.

In order to travel through time, the vehicle integrated with the flux capacitor needed to be traveling at 88 mph (140.8 km/h), and required 1.21 gigawatts of power (1,210,000,000 watts), originally supplied by a plutonium-powered nuclear reactor. However, for the time machine's return trip (from 1955 back to 1985), plutonium was not available, so a lightning rod was connected directly into the flux capacitor and was used while the vehicle sustained 88 m.p.h. 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 fueled by extracting hydrogen atoms from garbage.