It's 10.03pm. The Marty from the first film (called 1955 Marty here) is about to get sent back to the future. Cut to Doc at the clocktower. He's on it holding the cables. He watches the DeLorean from the first movie approach the clocktower. Doc finishes connecting everything and then slides down the rope.
Doc: Argh!
Cut to 1955 Marty in the DeLorean. The DeLorean hits 88mph and lets off blue flashes - it's about ready to travel through time.
1955 Marty: Doc!
Cut to Doc. Doc gets the cable out of the branch. The clock turns to 10.04. Lightning strikes it, and the lightning causes electricity to go down the cables, which Doc reconnects just in time, getting himself a slight electric shock as he does so.
Doc: Ahh!
The lightning enters the flux capacitor. 1955 Marty gets sent back to 1985. Doc dances with delight because the experiment was successful.
Doc: Yoo! Ha, ha, ha!
And then he turns to the Courthouse. After looking at it, he turns to his car - and 1985 Marty (the one we've been following through Part 2) comes running up to him.
Marty: Doc! Doc! Doc!
Doc: What?
Marty: Doc!
Doc turns and sees Marty.
Doc: Argh! A ghost!
Marty: OK, relax Doc, it's me, it's me, it's Marty!
Doc: No it can't be you...I just sent you back to the future!
Marty: I know, you did send me back to the future, but I'm back, I'm back from the future!
Doc: Great Scott!
He faints, and Marty bends down to tend to him.
Marty: Doc. C'mon.
CREDITS FOR PART 3.
Marty drives Doc home, back to Doc's mansion. He gets the still unconscious Doc out of the car and takes him inside.
Next morning Marty and Doc are sleeping. Marty is in the chair with his feet on the hoverboard, and he's put Doc on the couch. Doc's letter from 1885 is near the fireplace, drying. The TV has been on all night, and now programmes are just about to start as a show appears.
TV Announcer: (v.o) Hey kids, what time is it?
TV Kids: Howdy Doody time! (singing) It's Howdy Doody time, it's Howdy Doody time..
Doc gets up and clutches his head.
Doc: Great Scott! Ooh...Howdy Doody time?
Doc gets his tape recorder and starts speaking into it.
Doc: Date, Sunday, November 13th 1955 7.01am. Last night's time travel experiment was apparently a complete success. Lightning struck the clock tower at precisely 10.04pm sending the necessary 1.21 gigawatts into the time vehicle, which vanished in a brilliant flash of light leaving a pair of fire trails behind. I therefore assumed that Marty and the time vehicle were transported forward through time into the year 1985. After that...after that...I can't recall what happened. I don't even remember how I got home! Perhaps the gigawatt discharge coupled with the temporal displacement field generated by the time vehicle caused a disruption of my own brain waves resulting in a condition of temporary amnesia. Indeed I now recall the moments after the time vehicle disappeared ... into the future ... I saw a vision of Marty saying, "I've come back from the future."
During the next line, Marty awakens and gets up. Doc doesn't see him.
Doc: Undoubtedly this was some sort of -
Marty: Hey Doc. Doc.
Marty grins at Doc. Doc turns around, sees Marty and screams.
Doc: Argh! Argh! Argh!
Doc steps backwards from Marty, but the hoverboard is there. Doc trips over it and falls backwards onto his piano. His elbow lands on some keys which cause a sinister sounding chord to play.
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!"
Doc turns to Marty.
Doc: 1885! Amazing. I actually end up as a blacksmith in the Old West.
Marty: Pretty heavy, huh?
Doc turns back to the letter.
