For some reason, this scene always chokes me up a bit. Marty is realizing , to his horror, that the world he knew has gone down the tubes. His father is dead. One of the most vile people he's ever known is a rich, powerful man and is basically holding his mother captive.

"This can't be happening!"

He's hoping to wake up from this nightmare when the only voice of reason in this Hellish reality, Doctor Emmett L. Brown, speaks from behind him.

"I'm afraid it is happening, Marty. All of it."

Marty turns to face him.

"When I learned about your father, I figured you'd come here."

Doc has been pretty much a comical character through out the first film. Hearing him speak so solemnly was unexpected and, IMHO, very moving.