In the early morning hours of March 4, Doc Brown arrived at his secret lab in the temporal field generator Mk II. At this point, his memory was fully restored after a bump to his head activated the neurons that were scrambled by Officer Griff Tannen's electric pulse in 2035.
With the Mr. Fusion he had picked up in 2015, he headed to the site of the crash and took the tow truck. He drove the truck to the Lone Pine Mall, where he found Jennifer Parker and the second DeLorean, just moments after he and Marty had left for 2035.
He installed the Mr. Fusion, as well as the flux capacitor and the time circuits from the steam time car, into the second DeLorean. He then traveled with Jennifer to 2035, to rescue Marty McFly, who was forced to be left behind as they only had one diving suit, which was necessary as the temporal field generator Mk II offered no protection from the rigors and temperature variations of the space-time continuum.
- As Marty is stuck in the future, he wouldn't have married Jennifer, and he wouldn't have had his children, Marty McFly, Jr. and Marlene McFly, in this timeline.