Doc: I know, this is heavy.
They start walking off.
Doc: Marty, why are wearing that gun? You're not considering running against Tannen tomorrow?
Marty: Doc, tomorrow morning I'm going back to the future with you. But if Buford Tannen comes looking for trouble I'm going to be ready for him. You heard what that son of a bitch called me last night.
Doc: Marty, you can't go losing your judgement every time someone calls you a name! That's exactly what causes you to get into that accident in the future.
Marty stops and turns to Doc.
Marty: What? What about my future?
Doc: I can't tell you. It might make things worse.
Marty: Wait a minute, Doc...what is wrong with my future?
Doc: Marty, we all have to make decisions that affect the course of our lives. You've gotta do what you've gotta do. And I've gotta do what I've gotta do.
Doc walks off. Marty pauses, wondering what Doc meant by that, before following him. We cut to the railroad tracks that night. Marty is making some repairs to the tracks and Doc stoops down to talk to him.
Doc: Marty.
Marty: Yeah?
Doc: I've made a decision. I'm not going with you tomorrow. I'm staying here.
Marty: (slowly) What are you talking about, Doc?!
Doc: There's no point in denying it. I'm in love with Clara.
Marty: Oh man. Doc, we don't belong here! Neither one of us! You know it could still be you that gets shot tomorrow!
He shows Doc the picture again.
Marty: (continued) This tombstone could still be in your future!
Doc: Marty, the future isn't written. It can be changed, you know that! Anyone can make their future whatever they want it to be. I can't let this one little photograph determine my entire destiny. I have to live my life according to what I believe is right - in my heart.
Marty sighs.
Marty: Doc, you're a scientist. So you tell me. What's the right thing to do, up here?
Marty points to his forehead. Doc sighs. He knows what he has to do.
Doc: You're right, Marty.
Doc pulls a switch, and the DeLorean rolls itself onto the tracks. The tyres have been taken off the car.
Marty: Wow, that worked great.
Doc: I've at least gotta tell her goodbye.
Marty: C'mon, Doc, I mean, think about it - what are you gonna say to her, I gotta go back to the future? I mean, she's not gonna understand that, Doc. How long I been with ya and I don't even understand it? (pauses) Doc. Listen. Maybe we could...I dunno, maybe we could just take Clara with us.
Doc: To the future? (pauses) You've reminded me, Marty, I'm a scientist so I must be scientific about this. I cautioned you about disruption of the continuum for your own personal benefit; therefore I must do no less. We will proceed as planned, and as soon as we return to 1985 we'll destroy this infernal machine. Travelling through time has become much too painful.
Once again, Doc exits and Marty stares after him in confusion. We cut to later that night. A fire burns and Marty lies asleep next to it. Doc gets up and walks off. We then cut to Clara's house. Clara is inside writing at her desk when she hears a knock on the door.