Marty: Doc, calm down, OK? Just calm down, it's me, it's Marty.

Doc: No! It can't be you I just sent you back to the future!!

Marty: But I came back from the future, remember? Last night you fainted... I brought you home.

Doc: This can't be happening! You can't be here! It doesn't make sense! I don't believe you are here.

Doc runs into his bathroom and slams the door. Marty speaks to Doc through the door.

Marty: I am here, and it does make sense. I came back to 1955 with the you from 1985 to get a book back from Biff. So once we got the book back, you - that is the you from 1985 - were in the DeLorean when it got struck by lightning and you got sent back to 1885!

Doc opens the door.

Doc: 1885? It's a very interesting story, future boy, but there's one little thing that doesn't make sense. If the me of the future is now in the past, how could you possibly know about it?

Marty: You sent me a letter.

Marty shows Doc the letter. A few minutes later, Doc is reading it.

Doc: (reading) "Dear Marty: If my calculations are correct, you will receive this letter immediately after you saw the DeLorean struck by lightning. First, let me assure you that I am alive and well. I have been living happily these past eight months in the year 1885. The lightning bolt that hit the DeLorean caused a gigawatt overload which scrambled the time circuits, activated the flux capacitor, and sent me back to 1885. The overload shorted out the time circuits and destroyed the flying circuits. Unfortunately, the car will never fly again." (To Marty) It actually flew?

Marty: Yeah, well, you had a hover conversion done in the early 21st century.

Doc: Incredible! (re: letter) "I set myself up as a blacksmith as a front while I attempted to repair the damage to the time circuits. Unfortunately, this proved impossible because suitable replacement parts will not be invented until 1947. However, I've gotten quite adept at shoeing horses and fixing wagons!"