In the brig, Kirk freaks out on Spock for letting Sybok take over the ship. Spock, though apologetic for his seeming betrayal, says that he couldn't kill a "brother."
Kirk assumes that Spock is using the word metaphorically, but that ain't true. Spock and Sybok are half-brothers. Spock's mom is human, and Sybok's is a Vulcan princess—but they share the same daddy, Sarek.
Spock has never told Kirk any of this because he's never had any interest in discussing his personal life.
Uhura and Sulu enter the bridge. They tell a very confused Chekov that "Sybok will explain everything."
Sulu immediately starts plotting a new course, and Sybok enters the bridge. He starts whispering about "secret pain" to Chekov and casting him under the same strange spell as the others.
Meanwhile, Kirk is standing on Spock's shoulders, trying to find a way to climb through the roof of the brig. It's an utter failure.