The Back to the Future films twist the Hill Valley timeline into a pretzel, leading to four DeLorean time machines being present in 1955 at once. For all of Doc Brown's (Christopher Lloyd) talk about trying to avoid a paradox, one would think he and Marty McFly (Michael J. Fox) would've been a little more careful about their vehicle storage. Then again, through the magic of stable time loops, they actually wouldn't have been aware of all four cars at the particular time they were all present in 1955.
Time travel stories are always challenging to write, and that goes double for the Back to the Future movies, which sees two different versions of 1985 and sees Doc and Marty travel back to a 1955 that Marty had already traveled back to before. It's enough to make one's head spin if thought about too much, but at the end of the day, the Back to the Future trilogy is way more interested in being fun and entertaining than keeping its timeline straight.
Of course, that doesn't stop fans from noticing odd circumstances, such as the presence of four DeLoreans in Hill Valley on Saturday, November 12, 1955. The time machines coexisted for about five hours total, and here's how they all ended up there at that point.
Back to the Future: All 4 DeLoreans In 1955 Explained
The first DeLorean to be present in Hill Valley on November 12, 1955 had actually been there for over 70 years, hidden by Doc Brown in the Delgado Mine after his arrival in 1885. Another DeLorean, the one that transports Marty McFly back to 1955 in the first Back to the Future, arrives in Hill Valley on November 5. A third DeLorean time machine shows up in Hill Valley on November 12, after 2015 Biff Tannen uses it to travel back and give his younger self the sports almanac, which leads to the dystopian alternate 1985 timeline. This leads Marty and Doc to travel back to November 12 and take the almanac back from young Biff, creating the overlapping period where four different DeLoreans are all present in Hill Valley on November 12, 1955, beginning at 1:40 pm local time.
About five hours later, at 6:38 pm, the first DeLorean leaves Hill Valley, carrying old man Biff back to 2015, where he quickly vanishes after changing history. At 9:44 pm, the second DeLorean departs, when Doc is struck by lightning and sent back to 1885. A bit later, at 10:04 pm, Marty heads back to 1985 when lightning strikes the clock tower in the original Back to the Future, leaving a single DeLorean present in 1955. The Marty that was with Doc when he was struck by lightning retrieves that final DeLorean from the Delgado Mine and uses it to head to 1885 and find his friend on the morning of November 16.