Dexter took a trip to Nebraska this week because Deb told him Trinity returned and killed his wife and daughter. Dex knew this was impossible, of course, and figured Jonah did the deed. So, prompted by Brian the Ghost, he drove up there to kill Jonah.
But when he was ever so close to doing so, Jonah admitted he wanted to die. Turns out, his sister actually killed herself, largely because their mother blamed the kids for what happened to their father. Jonah then beat her to death in anger.
Despite Brian's prodding, Dexter just couldn't go through with it. He saw light in Jonah and he wanted to maintain it. So long, dark influence of Brian. Dexter drove home to end the episode.
Elsewhere, Masuka's intern created a program that narrowed Gellar's former students living near Miami down to a list of 200; while Travis told Gellar he was through with their arrangement.