The time bubble protecting the elderly Biff Tannen in 2015 pops, and he is erased from existence.

A time bubble is formed around time travelers when they travel through time, allowing them to make changes to the space-time continuum without being immediately affected by the changes themselves. Because of this, the ripple effect catches up with the time travelers last. Once the changes do catch up with the time travelers, the time bubble is said to pop, and the time traveler begins to fade from existence.

When Marty McFly traveled in the DeLorean Time Machine from to November 5, 1955, he accidentally prevented the moment where his parents, George McFly and Lorraine Baines met and fell in love. His siblings, Dave and Linda faded from existence first, as evidenced by their disappearance from the McFly family photograph that Marty carried with him in his wallet. The time bubble protected Marty and the photograph for a week, until it popped on November 12, 1955 at the Enchantment Under the Sea dance. Marty began to fade from existence, but George and Lorraine kissed, which restored the time stream. Marty and his siblings were brought fully back into existence.

Biff Tannen stole the Grays Sports Almanac after Dr. Emmett Brown, Marty, Jennifer and Einstein traveled to October 21, 2015. He also stole the DeLorean time machine and traveled back to November 12, 1955 to give the book to his younger self so he could make a fortune from betting on sporting events. His plan succeeded, but he had unintentionally changed time, arriving in an alternate timeline where he was no longer alive by 2015. As he traveled past the moment he died, the time bubble protecting him popped, and he faded from existence. Marty, Doc, Jennifer, Einstein, and the DeLorean time machine were protected by the time bubble, and were able to travel back in time to the alternate 1985 that Biff had created.

In this alternate timeline, Biff controlled Hill Valley, California, the alternate Marty had been sent to a boarding school in Switzerland, and the alternate Doc Brown had been committed to the asylum on May 2, 1983.

On October 26, 1985, in the timeline where the alternate Emmett Brown had been committed, Doc disguised himself and traveled to the asylum. He bribed the asylum nurse to talk with his alternate self. He realized that he had created a paradox, as he had arrived in a timeline where his alternate self had not yet created a working time machine. He realized that he and Marty were protected by a time bubble, but worried about what would happen if that bubble were to pop. He hoped to gain some insight into the timeline that he had entered. When he arrived at the cell he discovered that, at some point after being committed, his counterpart had been lobotomized.

Doc knew that he had to fix the timeline before the time bubble protecting them popped, so he went back to Marty to have him confront Biff to discover when he received the almanac. They traveled back to November 12, 1955 to get the almanac. While there, they discovered that the Marty that first traveled from October 26, 1985 was still there, as well as the elderly Biff Tannen, both temporarily protected by a time bubble as the events that allowed them to travel to the past had never occurred in this timeline.

After they got the book back from Biff, the DeLorean was unexpectedly hit by lightning. This sent the DeLorean, with Doc inside, back to January 1, 1885. Marty was stranded, but he received a letter from Doc, which had been sitting at the Western Union office for 70 years with instructions to deliver it to Marty at that precise moment. Marty then went to Doc's younger 1955 self to enlist his help. On November 14, 1955 Doc and Marty blasted into the Delgado Mine to retrieve the DeLorean. While there, they discovered Doc's tombstone and learned that he was shot in the back by Buford Tannen. Marty took a photograph of the grave, and decided to defy Doc's wishes given in the letter to go directly back to 1985 and destroy the time machine — electing instead to head back to September 2, 1885, the day after Doc wrote the letter, in order to pick him up and bring him home.

While in 1885, a time bubble protected the photograph, as it changed with information about who was killed by Buford Tannen. It changed from reading "Emmett Brown", to showing just a blank tombstone, to reading "Clint Eastwood" (the name Marty took in 1885), then to showing the grass without the tombstone. The latter wouldn't be a picture that would make sense for Marty would take, so the time bubble allowing the photograph to exist hadn't yet popped.

When Marty traveled back from 1885 to October 27, 1985, Jennifer Parker pulled out a fax that she had taken from the McFly residence in 2015 showing that the middle aged Marty had been fired from his job from CusCo. The message disappeared, but the fax paper remained. Like the photograph of the tombstone, it wouldn't make sense for Jennifer to have grabbed a blank fax in 2015, so the time bubble allowing the fax to exist in 1985 hadn't yet popped.

After Doc sent Marty back to the future and to the past, Doc spent all of his time perfecting time travel. Within seven years, he had already built a prototype time machine, the temporal field capacitor.

On October 24, 1962, Doc told the United States military about the device, 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 that version of Doc was erased from existence.

In the new timeline, Doc had read the letter from his alternate self, which was temporarily protected by a time bubble, allowing it to remain despite the version of Doc that wrote the note ceasing to exist. Doc hid from sight in his garage, with Copernicus, on October 24. The military personnel came and left in the new timeline without ever talking to Doc. Doc used the insurance money to fund the rebuilding of the temporal field capacitor, as well as his future inventions.

When Marty traveled from May 14, 1986 to June 13, 1931 he delivers a subpoena to Arthur McFly, Marty's grandfather. Marty's actions cause Arthur McFly to be shot and killed by the gangster Kid Tannen. As Arthur is dying on the steps of the Hill Valley Courthouse after being dumped there and left for dead, the time bubble protecting Marty pops and he begins to fade from existence, along with a photograph of his father. He traveled back a few hours to rescue Arthur from the El Kid speakeasy. The timeline was restored, Marty and George were brought fully back into existence, and Artie went into hiding.

However, when Marty and Doc returned to 1986, they discovered that they arrived in an alternate timeline where Kid Tannen was never arrested and the Tannen Mob controlled Hill Valley. In this timeline, Marty was run out of Hill Valley in 1984, so he was never able to travel back to 1931. Therefore, Marty and Doc traveled back to 1931 to ensure that Kid Tannen was arrested before the time bubble protecting them pops.

They succeeded in their goal to have officer Danny Parker arrest Kid Tannen, as he had in the original timeline, however their inclusion of a young Emmett Brown in the plans caused Edna Strickland to fall in love with him due to his righteousness.

So, when Doc returned to 1986 with Marty and Einstein, they returned to an alternate timeline where Edna and Doc got married, and Doc never invented the time machine. This caused the time bubble protecting Einstein and Doc to be popped, as like Biff before them, they had traveled past the time of their death as they didn't get the rejuvenation treatments that allowed them to live past their natural lifetime in this timeline. Marty crashed the time machine into a billboard, and it crashed to the ground, making it inoperable. Marty and the DeLorean are temporarily protected by a time bubble, and Marty went to the Emmett Brown in this timeline to help repair it. This Doc went by the name Citizen Brown, and ran Hill Valley with his wife Edna, using his inventions to keep the citizens docile and obedient.

Marty saw a picture of the Doc Brown that he rescued in 1931 and himself in a picture on Citizen Brown's desk. Like the presence of the elderly Biff and Marty in 1955, the time bubble temporarily protected them, allowing them to be in 1931, despite the fact that the events that allowed them to travel to 1931 never occurred in this timeline. This convinced Citizen Brown that his alternate self really did invent a time machine, and he fixed the DeLorean.

Marty and Citizen Brown traveled back to October 12, 1931. Marty convinced Edna that Emmett wasn't as righteous as she believed, and she broke up with Emmett. This restored the timeline, before the time bubbles protecting Marty and Citizen Brown popped, but it left both Edna and Emmett heartbroken. Marty convinced Emmett to focus on science, and Citizen Brown, who still has feelings for Edna, turned against Marty and told Edna that he was a Russian spy named Yakov Smirnoff.

Citizen Brown kidnapped his younger self, with the intent of getting him back into a relationship with Edna. He was convinced that Edna wasn't the problem, but rather it was his science that made things so bad. He hoped for his younger self to turn to other ambitions, such as following in his father's footsteps and taking up law. Marty had to rescue Emmett to ensure that he stayed a scientist so that he invents the time machine.

Marty succeeded in rescuing the young Emmett and revealing Edna to be the arsonist who burned down the Hill Valley speakeasy. Citizen Brown realized that Edna had always been power hungry, as he faded from existence when the timeline was restored and the time bubble protecting him popped.

Edna stole the DeLorean time machine and headed back in time. Stranded, Marty gave Emmett a clipping from the Hill Valley Telegraph that he had been carrying, telling him not to open it until he received the key to the city. The Doc Brown from 1986 in that timeline traveled back to 1931, after realizing that his friend Marty was the person who gave him the newspaper clipping when he was a teenager.

Shortly afterward, Hill Valley completely disappeared, and William McFly pulled up in his delivery truck. Marty and Doc realized that Edna had done something in the past to destroy the city, and asked William for information that might help them. He led them in the direction of Mary Pickford, who turned out to be an elderly Edna Strickland.

They learned that Edna accidentally burned down Hill Valley when she tried to burn down the Palace Saloon as she had the Hill Valley speakeasy. They traveled back to July 17, 1876 to stop Edna, and bring her back to 1931, before the time bubble protecting all of them popped.

Edna escaped in her DeLorean, but Doc was able to control her DeLorean remotely when Marty installed flux overrides on her DeLorean's flux emitters. They returned her to the Hill Valley Police Department on October 13, 1931. She was arrested for arson just before the time bubble protecting her DeLorean popped, causing it to be erased from existence.

When Doc and Marty returned to 1986, it was an alternate 1986, where Doc was living with Clara Clayton part time in Hill Valley, Erhardt Brown set up a scholarship for young scientists, and Kid Tannen and Edna Strickland married, making Edna the stepmother of Biff. They decided to leave well enough alone and headed back into Doc's garage. They were interrupted by three future versions of Marty, all who were prevented from being erased from existence by the temporary time bubbles which protected them. They wanted Marty and Doc to sort out the timeline to decide which future was the correct one. However, Doc took the Marty from 1986 into the DeLorean and left, letting him know that the only Marty that mattered was the one in the present. The time bubbles protecting the future versions of Marty would pop and Marty could handle that problem once he got there himself, as his future hadn't yet been written.

In 1991, Doc and his sons, Jules and Verne, headed back to the Cretaceous Period. They arrived just as the meteor that caused the extinction of the dinosaurs hit the Earth. Doc used his new invention, the Proprietary Ultrasonic Subatomic Molecular Redistributor, to disintegrate the meteor. They traveled back to 1991, and discovered that they had arrived in an alternate timeline where dinosaurs had evolved in the place of humans, making civilizations similar to those that humans had made in their own timeline. They played a video of Marty McFly playing guitar and saw him fade from existence. They had to return to the Cretaceous Period to make sure the meteor hit the Earth, before the time bubble protecting them popped, and they too would be erased from existence. As he was fading from existence, Doc used his Proprietary Ultrasonic Subatomic Molecular Redistributor device to reform the meteor, which corrected the timeline and caused them all to be completely brought back into existence. They headed back to 1991 just before the meteor hit the Earth.

Later that year, Jules, Verne and Marty travel to Wyoming on March 3, 1850, a date that has been carved on a wagon buckboard as the day that Daniel Clayton and Martha O'Brien married while on the Oregon Trail. Martha fell in love with Marty, and, in 1991, the inscription on the wagon buckboard was erased from existence. Doc realized that time had been changed, so he asked Clara for the date, and headed to the past to correct time. Jules and Verne were temporarily protected by a time bubble, but they had to get their grandparents together or they would be erased from existence.

Martha and Verne were kidnapped by an outlaw named "Wild Bill" Tannen. They escape, but find themselves having to fend off a grizzly bear. Doc realizes that they are in the area that would become the Yellowstone National Park, and recognizes the Old Faithful geyser.

Under Doc's instruction, Verne and Martha throw a buffalo hide over the geyser, and mere moments later, a column of water elevates them up and out of danger. Doc and Daniel fly in with a homemade glider. When Daniel comes to Martha's rescue and their eyes meet, they fall in love, and the wagonmaster conducts their wedding that evening. Jules, Verne, and Clara were brought fully back into existence, so everyone heads back to 1991 as the time stream had been restored.