The first two episodes deal with Dexter grieving and adjusting to the loss of his wife, Rita, who was killed at the end of Season Four by the Trinity Killer. He feels he needs to do some killing, to work through it, and singles out a dead-anmal picker upper who also kills young women. However, when he kills the guy in his house he discovers that one of the victims isalive--and has seen him do the killing. But Dexter can't bring himself to kill her--and ends up keeping her captive for time, until she settles down. This is Lumen (Julia Stiles). She wants revenge on he people who had abducted, raped and tortured her. And Dexter, seeing a somewhat kindred spirit, is willing to help her make her dream come true.
While all this is happening he's also trying to raise his young son, Harrison, who he thinks may be following in his father's footsteps when he hears words like "Die Die" come from the baby's mouth. And Quinn, who is his sister's partner at the police force, thinks that Dexter may have something to do with his wife's death. He eventually hirs corrupt cop Stan Liddy (Peter Weller of ROBOCOP) to spy on Dexter--and Liddy sees some unsettling stuff between Dexter and Lumen. However, Quinn falls in love with Debra and knowing how much he brother means to her wants to drop this informal investigation--but Liddy thinks busting Dexter will get him back on the police force.
I didn't find this season nearly as engaging as the previous four, primarily because the whole rape/revenge scenario has been done so many times. The whole serial killer vibe definitely took a backseat. And the main villain, Jordan Chase, pales in comparison to the Trinity Killer. He's just annoying and is dispatched very quickly. So while I enjoyed Season Five I felt a bit let down.