For such a memorable cinematic friendship, it's strange that nowhere in the course of the "Back to the Future" trilogy do we learn how Doc and Marty met in the first place. There are a few obvious questions about how they got so close despite clear differences in age, lifestyle, and general interests. But after the events of the films, there are several more specific, paradox-related questions about what exactly Doc knows when he meets Marty for the "first" time.
In 1995, Doc has now sent Marty off in a DeLorean time machine twice in a row, and in the process learned an absurd amount of detail about his own future. He knows he'll invent time travel someday, he knows how to prevent his own death in 1985, and he knows all about Marty's future for the next several decades as well.
Since Doc is not the most subtle operator, how will he play all of this casually when he meets the Marty of the '80s? Will he just pretend that he doesn't remember anything for the entire length of their friendship, or will he do a lot of drugs in the '60s and forget most of it? By the logic of the films themselves, Doc and Marty truly are "friends in time," as their relationship itself is a chicken and egg type paradox.
