<P> In the middle of the season, Debra finds out that Quinn, who believes that Dexter was involved in Rita's murder, has been investigating Dexter behind her back . Debra is very upset at Quinn and confronts him . Quinn decides that his relationship with Debra is more important than his (supposedly) unfounded suspicions about Dexter, and stops his investigation . At the end of the season, they have reconciled, and attend Dexter's son Harrison's birthday party together . </P> <P> At the start of the season, Quinn proposes to Debra, but she rejects him and ends their relationship . The ensuing awkwardness is made worse when she is promoted to Lieutenant, thanks in part to LaGuerta's political machinations . Debra earns the respect of the office, but struggles with the idea of some people not feeling' open' to her anymore because of her new job . The job is also very stressful in itself, and she comes to doubt whether she can handle the responsibility . Her work and personal troubles weighing down on her, she begins to attend therapy . Matthews attempts to make a deal with her about covering up his involvement in the death of a prostitute, but before she can take any action, LaGuerta tells Matthews' superiors about his indiscretion and lets Debra take the fall for it . At the end of the season, her therapist helps her realize that she has been in love with Dexter all along . She finally makes peace with the idea--on the same day that she catches Dexter in the act of murdering "Doomsday Killer" Travis Marshall (Colin Hanks). </P> <P> In the season premiere, Debra witnesses Dexter murder Travis Marshall . Dexter claims that he went to collect evidence, and was surprised by Marshall; he then says that he "snapped" and killed him on impulse . Debra reluctantly helps him burn down the abandoned church where he killed Marshall, destroying the evidence of the crime . Later, Dexter comes home to Debra, who is surrounded by Dexter's victim's blood slides, a pack of knives, and other tools that Dexter uses to kill . Debra reluctantly asks Dexter if he is a serial killer . Dexter, taken aback, replies yes . Horrified, Debra recoils from Dexter as he admits that he is the "Bay Harbor Butcher", and that Harry taught him how to get away with murder . </P> <P> After reading Lundy's files on serial killers, Debra says she wants to help Dexter, and make sure he doesn't kill again . Dexter agrees, even though he doesn't really have a choice, as Debra (probably) would have arrested him . Dexter slips away from Debra long enough to dispose of Ray Speltzer (Matt Gerald), a serial killer she had tried and failed to send to prison . Debra admits that she understands why Dexter takes the law into his own hands, and tells him that she will not stop him as long as he does not tell her about it or interfere with Miami Metro investigations . She even compromises her own professional ethics by falsifying evidence to mislead LaGuerta, who is trying to reopen the Bay Harbor Butcher case . </P>

What season does dexter's sister find out he's a killer
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