Doc’s secret lab was a laboratory that was kept a secret even from Emmett Brown's assistant, Marty McFly. In 1986, Marty received a note from Doc’s wife Clara, which stated that Doc traveled from 1893 to some point in the future and never returned.
Marty and his girlfriend, Jennifer, first went to Doc's garage to see if they could find out anything, but it was sealed up tight. Jennifer realized that, since Marty never saw Doc build the DeLorean time machine, he must have another lab somewhere. They went to Goldie Wilson Jr. for help at the records office. Doc registered the location under his birth name, Von Braun.
When they got there, they saw that Doc kept many of his inventions here, including the temporal field capacitor and another DeLorean DMC-12, which had not yet been converted to a time machine. Marty decided to write a letter to Doc to let him know where and when to find him. His plan worked, as Doc appeared in the lab immediately afterward, in a diving suit, with no memory of who he was, when or where he was, or even who Marty and Jennifer were.
After they decided to find his time machine, his memory slowly began to be pieced back together. He knew that Marty had left the letter, but didn’t know why he thought he was in danger. Doc also revealed that he had built a time machine in-between the DeLorean time machine and the Jules Verne Train. The steam time car was not sturdy enough for too many trips, so Doc had intended to travel to the future to get what he needed and then go back to 1893 to complete the time train. However, he ran into trouble with Griff Tannen in 2035 and lost his memory in the process.
Doc revealed that he had come up with a method for time travel without a machine, in case the steam time car couldn’t make the trip. The time parachute would take the time traveler up to a large distance, and then release the occupant. The time traveler would then fall, and when a speed of 88 m.p.h. was reached, time travel would occur as the parachute was attached to a flux capacitor and time circuits. The parachute would then be deployed, and the time traveler would land safely in the new time period. With this knowledge, Marty told Doc that he was going to go with him to 2035 to find out what happened. To protect themselves from the stress of space-time and the extreme temperature variation, Doc donned his diving suit, and Marty donned a radiation suit.