The temporal field generator Mk II was Doc Brown's rebuilt and improved version of his temporal field capacitor. Like the original, it only allowed time travel during the lifespan of the device itself. However, it allowed time travel to the past without causing an overload of flux energy, and allowed for the transport of larger objects.
On October 24, 1962, Doc told the United States military about his temporal field capacitor, as he needed funding for his inventions. He later had worries that he could be ushering in an arms race through time, coupled with his regrets about ushering in the nuclear arms race of the Cold War through his participation in the Manhattan Project.
The next day, he wrote himself a letter about his plans, and purposefully sent the letter back several months in the past, to August 1, when he knew that he and his dog, Copernicus, would be safe asleep in his fireproof garage. The Brown family mansion burned down, and the military personnel came and left in the new timeline without ever talking to Doc. Doc used the insurance money to fund the building of the temporal field generator Mk II, as well as his future inventions.
On March 3, 1986, Marty McFly and Jennifer Parker discovered Doc's secret lab. While exploring the lab, Marty absent-mindedly flipped a switch on a device he didn't recognize and backed up when it loudly turned on. This device was the temporal field generator Mk II, which remained active for nearly fifty years.
On September 16, 2035, Doc Brown and Marty McFly used hoverboards to escape Officer Griff Tannen. Officer Tannen followed them to Doc's lab, where Marty informed Doc about the device he turned on five decades prior. Realizing that he had a way to escape Tannen, Doc donned his diving suit to survive the stresses of space-time and the temperature variation that occurs during time travel.
Doc traveled to 2015 to pick up a Mr. Fusion, then headed to 1986 to pick up Jennifer Parker, who had found an unmodified DeLorean DMC-12 in Doc's secret lab. Doc outfitted the DeLorean DMC-12 with the Mr. Fusion as well as the flux capacitor and time circuits from the time parachute that he carried in his backpack. Jennifer then drove the new DeLorean time machine to 2035 to rescue Marty from Griff Tannen.