I think I forget what being a 16 year old girl is like. Through quite a bit of this book, I was sighing at Tris and rolling my eyes. But as it progressed, I got it.
I had a rocky upbringing but obviously wasn't taught to battle, use guns and take over Erudite headquarters as a 16 year old. I didn't watch my parents die bloody deaths and I didn't have to kill my best friend. (thank goodness).
Tris is now having panic attacks when she has to hold a gun (don't blame her there) and seems to be recklessly throwing herself in the path of death and mayhem for no particular reason, although she'll be happy to give you all the reasons she made up. Naturally, Tobias (aka Four aka Tris' boyfriend) is getting fed up with all of this especially since they are in the midst of a deadly war between factions.
Spoilers, dear reader.
The Dauntless end up back at their home base when it's clear the other factions would rather turn them over than stick their own necks out. Sweet. Anyway, they attempt to blind all of the cameras that the Eurdite have placed in Dauntless headquarters but they miss some. That results in very young Dauntless being mind-controlled to commit suicide until a Divergent (such as Tris or Tobias) hands themselves over to Eurdite headquarters. Tris, suicide mission bound again, walks herself right in to hell. Good girl.
She's experimented on by the head of Eurdite and lo and behold, Tobias walks himself right into hell with Tris because, at first it was a "You die, I die" deal (very Shakespearean) but then it becomes clear Tobias has no intention of dying with Tris and is trying to find the control rooms of the headquarters for an attack later. Fortunately, he doesn't want Tris dead either and they do make it out.
From there, an attack on the Eurdite headquarters is planned by Dauntless and the factionless. Tris joins up with others to get there first because the data must be saved.
And I leave you there. Because it's quite a fiery, bloody end to the book which leaves you hanging and desperately cursing the library for not having enough copies of Allegiant. Not that I would curse the library.....