The Traitor takes place several years after The Son and begins on Tris's Visiting Day. Squee! I'm excited that Tris is in this one!
In this book we learn that Four has been suspicious of Max and Jeaniene Matthews for quite some time. He uses his position in the control room to spy on them.
More tension is building between Four and Eric, especially after the attack on Tris.
Four is going round and round trying to figure out the right thing to do. Trying to decide where his loyalties lie. And Tris is the one who makes everything clear. I loved it, my favorite! I really hope there's more to come!