 Before his departure, Peter recalls a line from a film. Why does it come to mind for him?
I do not think it was what Peter called an emotion; and yet it had nothing to do with logic, either. "The peoples of the world, then, know what they are, and know that we face the most momentous struggle in our history. They have treated us precisely as we, in less enlightened days, might have treated a newly-discovered race of lower animals. Peter took it in fifteen seconds, running, and stumbled to a halt in front of the door marked "Radiation." It was a street scene in the Science City of Manhattan, flooded by the warm spring sunshine.
 He recognizes that he will be a changed, mutated man when he returns. He literally will come back "not as a boy."
 He recognizes that he will be a changed, mutated man when he returns. He literally will come back "not as a boy."  He's trying to convince himself that he and humanity will be able to come back, with the emphasis on "We'll come back."  The situation is grave. Like men who go off to war, the journey will change them. He won't be coming home as the same "boy."  He's not sure he'll be coming back, and the song is bittersweet for him.