The spare DeLorean
While creating his time machine, Doc Brown had kept two DeLorean DMC-12 vehicles in his secret lab. One was modified with a flux capacitor, time circuits, and a nuclear reactor, while the other was not.
Marty McFly and Jennifer Parker found the other DeLorean DMC-12 in Doc's secret lab in 1986. Although it wasn't adapted for time travel, Marty and Jennifer took it to escape the police, who had been alerted when the alarm was tripped in the secret lab. Later, Jennifer drove it when chasing after Marty, who stole the tow truck that had Doc's steam time car hooked up to it, and was trying to outrun Needles. She used the DeLorean to ram Needles into a delivery truck loaded with boxes of eggs.
Marty, Doc and Jennifer then met at the Lone Pine Mall parking lot. Marty checked the chronometer of the steam time car, and determined that it had arrived from 2035. He donned the radiation suit that was in the DeLorean, and Doc donned the diving suit that was in the steam time car.
They took a hot air balloon up to the distance of half a mile, and then dropped out of it. Doc's time parachute was activated, sending Doc and Marty to 2035, crashing into the Lone Pine Fli-Drome during a fli-derb competition.
Minutes later, Jennifer ran towards the DeLorean when she spotted an incoming vehicle, but stopped when it turned out that it was Doc, driving the tow truck.
Conversion into a time machine
Doc had purchased a Mr. Fusion in 2015, and had brought the time circuits and flux capacitor from the time parachute. He installed all of these devices into the spare DeLorean, completing his second DeLorean time machine. Jennifer and Doc then headed to 2035 to rescue Marty.
When they had arrived, Commissioner Wilson had arrested Officer Griff Tannen, who had abused his police powers while pursuing Marty and Doc. She had also arrested Marty for his actions at the fli-derb, and told him that he'd likely get the same sage implant and personality rewrite that Griff Tannen had received.
Griff broke free and attempted to run them all over, when Jennifer and Doc knocked his hovercar out of the way. Marty got into the DeLorean and Jennifer attempted to outrun the police, who were in pursuit, along with Griff Tannen.
Doc informed Marty and Jennifer that every person in 2035 wore a suit outfitted with a hover technology protection that acted as a force field and made them bounce when hit. Knowing that the people would be unharmed, Jennifer sped full speed into a crowd of people. As the bouncing pedestrians blocked Griff's view, he did not see the sewage reclamation project ahead, and crashed into it. Jennifer sped up to 88 m.p.h and traveled back to 1986.
Doc dropped Marty and Jennifer at the McFly residence. Jennifer informed Marty that the steam time car, minus the equipment for time travel, was in the back of Marty's truck. Marty walked Jennifer home, then Marty returned to his own home. Before Doc left, he gave Marty a bag containing several canisters, telling him to leave a note in the secret lab if he ever needs help.
Doc returned home to 1893, and kept the second DeLorean in the livery stable, next to the Jules Verne Train. On September 7, the eighth anniversary of the day he met his wife, Clara Clayton, Doc traveled to 2017, along with his son Verne, who was hiding in the back seat. Doc forgot his anniversary, and used the trip to pick up a Jules Verne novel as an anniversary gift, as well as to pick up hover conversion equipment from Goldie Wilson Hover Conversion Systems.
Doc used the equipment he purchased from Goldie Wilson III in 2017 to have the second DeLorean time machine and the Jules Verne train hover converted after he returned home to 1893.
The temporal rivalry
Meanwhile, Marcus Irving saw Doc give Marty the canisters, and then witnessed the DeLorean break the time barrier to travel through time. Irving was a scientist who, in 1962, was contracted by the United States military to build a time machine after their first choice, Doc Brown, appeared to be an insurance thief when his mansion burned down that August. He was let go by the military in January 1986, as he could never accomplish the problem of excess flux energy.
Marcus was angry at Doc for not taking the military position, and therefore wasting over two decades of his life. On April 14, 1986, he broke into Doc's secret lab, and left a message in a canister. He then came face-to-face with Marty McFly, who was also looking for Doc in an attempt to find out why his memories did not reflect the current timeline. He informed Marty that he had built a partially working time machine using a Yugo, and warned him that his conflicting memories could mean he could be erased once the current timeline caught up with itself.
Marty sent a message to Doc, who arrived shortly afterward. While Doc and Marty were talking, Marcus secretly removed the tires from the DeLorean, and used Doc's flux capacitor to solve his problem with flux energy, which was the final step needed for time travel.
Marcus was afraid of altering time too much, so he had spent years driving around with the flux capacitor in his trunk, worrying that someone might steal it from him. Eventually, he decided to travel into the future. Since he was afraid of altering time too much, he decided to only time travel in "baby steps", so he could better prevent any major changes to the timeline. He decided to take 2008 technology back to 1997 in order to make himself rich.
By this point, he had upgraded his Yugo with steel panels to help with the dispersal of flux energy. He also upgraded the tires, as the polymers used in 1980s rubber tires weren't suited for the rigors of the space-time continuum. Once he began his plan, he realized that he could be confronted by Marty and Doc at any time, as he viewed them as time cowboys, as, unlike himself, they weren't afraid to travel many years into the past and future, making large changes to the time stream.
Irving searched the internet for pictures of Marty and Doc from the 1980s. He then used those pictures to build several androids using futuristic parts and sent them back to 1986. Several of the Marty doppelgangers appeared soon after Irving had stolen the flux capacitor and kidnapped Marty. They tried to kidnap Doc as well, but he escaped in the DeLorean. It was still able to fly despite lacking tires, since the hover technology in the rims were untouched. One of Marty's doppelgangers held onto the DeLorean while it was in the air. This surprised Doc, as he thought that it would be very unlikely that Marty could have survived such a trip.
After fighting, Doc discovered that it was an android version of Marty, with a metal skeleton and lifelike skin and eyes. When the android was kicked out of the car, Doc returned to Marty to let him know that the situation was not as it appeared. He incapacitated the other androids with the stun gun that Griff Tannen had used on him in 2035. They then went to find the remaining android before it could do any harm, but they were too late.
The android knocked out Needles' gang with a knock out serum mixed with a memory clouding agent that made their memory hazy. It was doing the same to Needles himself, when Marty showed up to stop it. Doc meanwhile, headed to the DeLorean, and found that Irving had repaired it, placing the flux capacitor back in its proper position and replacing the tires.
Doc was going to use a remote control override on the android when the aged Marcus Irving used the knock out serum on him. The DeLorean was piloted by an android to 1997, and Marcus took Marty and Doc to January 8, 19654 B.C. in his Yugo. Doc feigned haziness from the serum, which really didn't affect him since the majority was used on Needles and his gang. This allowed him to lay a trap. Doc knew that Marcus would be afraid of major changes to the timestream. So, he told him, using slurred speech, that their existence in this era might wreck the entire ecosystem.
Marcus left them behind, but Doc was sure that he would return. Doc and Marty then jumped into the river to escape sabre-tooth tigers. Marty barely made it, getting scratched in the face. They climbed a rock, which Doc knew was still solid in 1986, and waited for Irving. As Doc had predicted, he was worried about the butterfly effect that they might cause, so he showed up in his Yugo shortly after they arrived on the rock.
Once Marty and Doc were in the Yugo time machine, one of the androids treated Marty's scratch and they traveled forward to 1997. They were taken to Marcus Irving's lab where he locked them in a room with his androids, telling Marty and Doc that he didn't mind if they were killed in this time period.
Doc used his remote control override, and the DeLorean drove over an android, out of the garage, and up to the penthouse level, where Doc and Marty were being kept, crashing through the window.
The elevator Marcus was on automatically takes Marcus down to the garage level, while Marcus is pushing the button trying to get it to go up. Doc and Marty get into the DeLorean and fly down to the garage.
Doc explains that they can't just go back and stop Marcus from taking the flux capacitor, since the effects on a time traveler aren't instant, pointing out that it took a week for Marty to begin to fade from existence in 1955. Marcus and Doc could get in a reverse time race, with each vehicle going further back in time to try to prevent the other from making changes.
Doc rammed the DeLorean into the elevator cord, slowing Marcus down. Marty and Doc then look at all the future technology. Marty sees a photo album and realizes that Marcus had been alone all this time. A female android that has been built to be Marcus Irving's sister comes online, and Doc and Marty realize that Marcus has become a bit crazy during his decade of living all alone.
Marty realized that his fear of being all alone is what drove him to find Marty in the first place. They decide to take Marcus to his younger self, traveling in the DeLorean back to 1986.
In 1986, Marcus Irving is traveling in his Yugo GX, happy that he has taken the flux capacitor. Marty and Doc arrive from 1997 in the DeLorean, followed by the older Marcus in the Yugo.
Marcus is happy at first that his older self has come back to stop Marty and Doc, until he realizes how imposing he has become. He sees the army of Doc and Marty androids and thinks that everything he told Marty about alternate dimensions was true. However, Marty tells him the truth: that he has become a lunatic, a kidnapper, and a thief.
A woman and her niece get caught in the middle of the ensuing chaos, as the androids are ready to destroy everyone around them. Marcus realizes that he has become a person that he no longer viewed as himself, as his older self is willing to commit murder.
Marcus slams the flux capacitor into the android that is about to attack the innocent bystanders. The older Marcus Irving realizes he was almost a murderer. As he fades from existence, he commented that he is happy for the man he once was, and the man he could become.
The androids and the steel plated Yugo time machine fade from existence, and Marcus tries to explain that he is a scientist and states that the androids that faded from existence were holograms.
The woman introduces herself as Gabriela Sanchez, and her niece as Maria Sanchez. Gabriela thanks Marcus for saving her life, and asks Marty and Doc who they are. Marty and Doc reply that they are Marcus Irving's friends. Which Marcus replies that they are probably his best friends, who helped him in a moment of crisis.
Needles arrives, and tries to harass Marty, but he wrestles with the unbelievable, and hazy, memories that he has of Marty fighting another Marty without a face. In light of this, he decides to leave Marty alone for a change.
Later, Marty is telling the story of the events to Jennifer while she is at work at Marty is telling the story of the events to Jennifer while she is at work at Shanerburger. Marcus tells Doc and Marty not to embarrass him, since he is on a date with Gabriella.
Marty and Jennifer wonder what will happen to Marcus, and Doc comments that he hopes he returns the Yugo to the authorities as not everyone can be trusted with unlicensed nuclear reactors.
Doc becomes a mentor to Marcus, as they both work to fix the second DeLorean time machine, both in matters of time travel as well as in dating etiquette, as Gabriella was the first woman Marcus had ever dated. After several days, Marcus and Doc had finished fixing the DeLorean, and Doc headed back to 1893 to return to his family.