The season four finale of Dexter was billed as a game-changed, and, wow, it didn't disappoint.
First, Deb confronted Dexter with the news that she found out his biological mother was Laura Moser and his brother was the Ice Truck Killer. But Dexter said he had no idea about any of that and it led to a very nice moment between Deb and Dex.
From there, Deb interviewed the abducted 10-year old boy and he described the Four Walls/One Heart logo on the van in which Arthur took him. This led the police to matching Four Walls volunteers with locales around the country where murders had taken place - and to Arthur Mitchell. They raided his house, but Arthur was gone. He was seemingly free, in his Mustang, when Dexter tracked him down and set him up on his table.
The pair bonded, like only serial killers can, and Arthur seemed almost at peace with his fate. Fate. He kept using that word, in fact. He kept saying this was all part of God's plan. Dexter said he didn't accept that and sliced up Arthur Mitchell like he was another victim. But we all know he wasn't.
When Dexter returned home, he and the viewers got the shock of a lifetime: Harrison was crying in the bathroom, covered in blood... and Rita was lying in a pool of blood in the bathtub. The episode, and the season, ended with Dexter carrying Harrison away, just as Harry carried away Dexter so many years ago.