Dexter is incredibly frustrated with Trinity and in his frustration he hits Quinn. He gets arrested after a small accident and a run in with the cops. In jail, he reflects on his life and realizes how big of a part Rita plays in his life and how much she affects him. Dexter decided to change his ways and become a new man. He needs to kill Trinity first to settle things.
Dexter was able to find Trinity. The SWAT team arrived at Trinity's house, but Dexter knew Trinity had already left. Meanwhile, Deb finds out the truth about Dexter's mother, which implies that she knows that Dexter's brother was the Ice Truck Killer. Dexter pretended this news was new to him and acted surprised. He is now concerned that Debra is getting closer to finding the truth about him, which gives him more of a reason to change his life.
Dexter finally catches up with Trinity and kills him; allowing Trinity to enjoy his toy train one last time. Dexter realizes he is not like Trinity. He is good for his family and there is a mutual feeling of love. He returns home with a new outlook on his life and his family, only to discover Rita dead in the bathtub with baby Harrison facing the crime scene; which is the same position Dexter was put in that turned him into a serial killer.