She's having a bit of a moment, so Tris plays hooky and leaves the Dauntless compound to go visit her brother in Erudite.
She takes the train downtown to the library that is Erudite home base.
Tris runs into brother Caleb (which is lucky, since the Erudite don't seem to like answering questions, like "Where's my brother?")
They go out to the nearby park to talk. (If you know Chicago, you know she's talking about Millennium park and the giant statue named "Cloud Gate"—but which everyone calls "The Bean".)
The family reunion is tense. (They could both ask each other, "So, how are things since you stabbed mom and dad in the back?")
But Caleb warns her that something big and wrong is happening soon. And Tris asks him to check out the simulation serum (which her mother asked her to do in chapter 15). It's an unhappy meeting.
And things don't get better. Tris is asked to go meet Jeanine Matthews, the head of Erudite. She's the woman who was plotting with Eric in chapter 21. She's the designer of the aptitude test, she wants to find Divergents, and she suspects Tris. What a fun talk this will be.
But clever Tris lies and says that she agrees with the Erudite reports about how bad Abnegation is, Jeanine lets her go.
After that, Tris is returned to the Dauntless. She gets scolded by Eric for leaving without permission.
Luckily, Four/Tobias comes in and helps Tris convince Eric that she just was embarrassed. It's not like she was plotting against Dauntless or anything.
Eric leaves (thank goodness) and Four/Tobias makes it all up to Tris: he ignored her at breakfast because it wouldn't look right for an instructor to be kissing a student. So, ta-da, everything is better; they arrange to meet later that night because Four has something to show her. Oh, get your mind out of the gutter.
Back in the dorm, Christina tells Tris that Will kissed her. So, yay, everyone is paired up. Stop here for a happy ending.
But if you keep reading (this really long chapter), you'll find that Tris and Four/Tobias take a train ride that night (with some kissing on the way). He points out that the Erudites are still awake at night; and that he found secret computer files about Dauntless waging war for Erudite, probably on Abnegation.