Matthew gives the specifics about the memory serum. It doesn't erase everything. People will still remember how to tie their shoes and flush the toilet (we hope), but they'll forget other things, like their name or their favorite food.
Tobias tries to convince Tris that this is a bad idea, but she resists him, basically saying that if he can think of something better, they'll do that.
They decide to go ahead with the plan, with Tobias and Christina heading into the city just in case the plan doesn't work. Tobias wants to confess to his role in putting Uriah in a coma, and Christina wants to inoculate her family against the memory serum.
Tris goes to Nita and basically says: "Tell me about the security measures blocking the Weapons Lab."
Nita tells Tris that if she doesn't know the passcode, an aerosol death serum will spray and kill the person trying to break in.
Tris says she'll set Nita free if the plan works, so Nita agrees to help, telling Tris to get some explosives from Reggie. And boom goes the dynamite.