Tris sneaks out in the middle of the night, tells Tobias "I love you" (28.5) while he's sleeping (which, by the way, totally doesn't count), and tells Christina that she should persuade Marcus to cooperate.
Christina wonders if Tris is going to turn herself in to Erudite. ("Oh, no, it's not like I'm practically making arrangements for what people should do after I die. I don't want to use the word will around you, Christina, because it might remind you of Will. Your dead boyfriend. Who I shot. Oh, oops. Sorry.")
After that, Tris walks to Erudite headquarters and turns herself in.
Peter comes down to collect her and take her to her cell.
Tris wishes she could just hold her breath until she died, and she's super disappointed that the human body just doesn't work that way.