It turns out that Will's older sister, Cara, betrayed Erudite by evacuating the loyal Dauntless when Erudite came to attack the Divergent.
Tris examines where she was shot and finds a metal disc instead of a bullet.
The metal disc is attached to a needle, which has injected blue dye into her body. What are they trying to do, tattoo people by gunshot?
No one has any idea what's going on. They just know that Jeanine is planning something. ("What could Jeanine be planning? The same thing she plans every night: to take over the world.")
Tris spends all night removing needles from people.
Tris finds out that Caleb is still alive (yay!), and so is Marcus (*crickets*), and so is Peter (boo! hiss!), who fled to Erudite.
Tris and Tobias try to figure out just what the heck Jeanine is up to.
Tris and Tobias think that they've been injected with a simulation transmitter, allowing Jeanine to control them from afar. Basically, it's like being able to brainwash someone via the Clapper.
Tobias then criticizes Tris for being careless. "I don't want to lose you" (17.147), he says. She's too suicidal (17.148) to really care what he thinks.
Tris and Tobias go to the cafeteria (these kids have lunch break more often than Bella does), where Caleb talks about how he can't go back to Erudite. He'll be factionless, and… ooh! Cake and ice cream.
Tris falls asleep while everyone else talks around her. We'd rather be eating cake and ice cream than talking to her, anyway.