Nita is miffed when Tobias brings Tris along, especially since he was supposed to be "protecting" her.
Tris thinks Nita just used that whole "protect Tris" spiel to manipulate Tobias.
Tobias is shocked—shocked!—that he's been manipulated, thinking: "I can usually rely on myself to see a person's ulterior motives" (25.5). So says the boy who has been successfully lied to by his mother, his father, his girlfriend…
Nita escorts Tris and Tobias to a storage room, where some dude named Reggie shows them a bunch of horrible pictures illustrating the way people were treated during the Purity War.
Nita says that the whole attack simulation catastrophe from the end of Divergent was organized by the Bureau… they're the ones who slipped the serum to Jeanine. Holy plot twist, folks.
Now the Bureau has an even more nefarious plan: they want to use the memory serum to just reset everyone.
Nita's plan is to steal the memory serum.
Tris isn't buying it. She thinks Nita is planning a lot more than just stealing the memory serum. (Like what? Tying everyone's shoelaces together while they sleep?)
Tobias agrees to help. Tris is like, "I'm out!"—and they argue. Tobias never trusts Tris, Tris jumps to conclusions… it's the same argument they have in all the prime-numbered chapters.
Just as all these arguments usually end, this one ends when Tobias—say it with us, Shmoopers—stomps out of the room.