We pick up where Part II left off.
1955 Doc (Christopher Lloyd) has just finished helping Marty's counterpart return to 1985, and is running down the street laughing.
Marty runs to him and grabs him from behind.
Doc is shocked, saying that he just sent Marty back to the future.
Marty says that he's back from the future, and Doc faints.
Doc awakes back at his family mansion with the TV playing Howdy Doody, not quite sure how he got home.
Marty is nodding off in a chair, resting his feet on his hoverboard.
Doc gets out his recorder and swears that he may have seen a vision of Marty afterwards saying he had 'come back from the future,' but thinks he may have been hallucinating.
This turns out to be a reality, when Marty appears, scaring Doc.
Marty explains that he came back to 1955 in regards to recovering the sports almanac from Biff to save the future.
Marty then explains that afterwards, the 1985 Doc was in the DeLorean when it got struck by lightning, which sent him back to 1885.
Doc questions how Marty would know this, when Marty reveals the 70-year-old Western Union letter he received.
Doc reads over the letter.
His 1985 version notes that the DeLorean has been hidden in the Delgado gold mine, with instructions for replacement parts for the 1955 Doc to help repair the time machine.
The letter also notes that the flying circuits were fried, meaning the DeLorean will never be able to fly again.