According to a fan theory, Marty McFly dies in alternate version of Back to the Future's timeline. Biff has profited off the sports almanac, given to him by his future self, which allows him to bet on sporting events and definitely win, and Marty’s father has died.
Back to the Future II shows the effects of meddling with the timeline and the importance of avoiding other versions of oneself. The sequel runs parallel to the plot of 'Calvin Klein') and Marty’s then-teenage parents.
The theory (via Doc saves his friend Marty by going back in time, and this is how he knows when and where to drop the flags. According to the theory, Doc doesn’t tell Marty any of this as he has already shown himself to be emotionally fragile, and Doc wouldn’t want him to have to deal with his own death.
The theory is plausible, as Doc and Marty have changed the timeline multiple times by that point, so it wouldn’t be completely unbelievable that Doc does it again with no repercussions. However, Doc going back in time again would mean that there are three Doc Browns in 1955, two of them specifically following Marty and Biff's car down the tunnel, and both in a flying DeLorean turned time machine. As Doc tells Marty, meeting your other self can have disastrous consequences, and it would be impossible for Doc to save Marty without the other Doc seeing him.
Also, Doc gets Marty to Biff’s car on the hoverboard before flying above them and following the chase. When Biff turns around at the end of the tunnel, he has already come out the other side, so Doc would’ve easily seen him turn around. Biff waits a while, revving his engine to chase Marty, and this gives Doc plenty of time to realize Marty is being chased, allowing him to get to the other side to rescue him. He tells Marty that he has a plan, so there is no doubt that Doc is following them closely. Therefore, he could pull off the rescue without Doc breaking time travel rules.
The theory of Marty’s death in Back to the Future II seems possible at first. However, the logistics of the scene show that Doc has time and a clear vision as to where Marty would be and when without having to risk going back in time again. Doc would be unable to avoid his other self, and he would unravel the fabric of the space-time continuum, destroying the universe, which makes it unlikely for this theory to be possible.
