Cassie Sullivan returns to get her little brother, Sammy, and takes him to safety, just like she promised
We take a left here!
She finds that he has grown up. She finds this because when she gives him his bear that he has had since he was a baby and loaned to her (and told her to keep him safe, which is what she's been doing this entire time), he refuses it.
As the get ready to leave (because the place was about to blow), Cassie runs into a former crush of hers, Ben Parish, who has returned to get Sammy (or Nugget, as he calls him) as well. They start fighting because they think that the other is trying to kidnap Sammy.
They both work out the fact that they are here to help, although they still don't trust each other. Ben knows a way out since he went to that military school, Camp Haven.
They are almost there when they are caught by Colonel Vosch, the commander of Camp Haven. He takes them as captives.
He takes Ben and Cassie to a room where they are stationed in two chairs in front of a mirror, where they can see Sammy hooked up to a machine and a soldier hovering over a button that says EXECUTE...