The scene picks up right after the scene in which Kirk wakes up in the hospital bed. Spock motions to McCoy that he should join him in the hallway for a moment, and they step out.
MCCOY: Yes, Spock?
SPOCK: One thing has been bothering me the past few weeks. The other seventy-two of Khan's crew were all possessed of his enhancements, correct?
MCCOY: With a few variations here and there, but essentially, yes.
SPOCK: And the essence of importance, the healing factor. They all had it.
SPOCK: So, you didn't actually need Khan alive to save the Captain, did you?
MCCOY (wry grin): No, Spock. I needed Khan alive to save you.
(Cut to Kirk's memorial speech about not becoming as bad as those we fight.)