Jennifer Carpenter and Michael C. Hall

Now that Deb knows her brother's big secret on Dexter — that Michael C. Hall's titular character is a serial-killing blood spatter analyst — it's the beginning of the end for the Showtime series.

At Monday's Television Critics Association fall TV previews, Hall reiterated what executive producer Sara Colleton told TVGuide.com earlier this summer: The eighth season of Dexter will be its last. (Earlier in the day, Showtime chief David Nevins insisted there was still a possibility of the series carrying on.) 

Dexter Season 7 Scoop: How will Deb react to Dexter's secret?

"[Deb] finding out does make an endgame feel a little more palpable and imminent," Hall said. "The plan is to do this season and a final eighth season. To really tell this story of the two of them negotiating their relationship in this new landscape requires that much time."

In the Season 6 finale, Deb (Jennifer Carpenter) rushed to the church to profess her love for Dexter, only to find him plunging a knife into the heart of the Doomsday Killer (Colin Hanks). The seventh season will pick up right immediately from the finale, with Deb confronting her brother — something Carpenter has been dying to play. "It's the strangest, scariest seduction scene, in a weird way," she said. "I have been looking forward to it. I was very curious about how they would write it and also terrified. I didn't want Debra to lose any sort of credibility with the audience. I think it was imperative that [she] finds out while [she] was already trapped in this web of acceptance for him."

"Without a doubt, it's the most fundamentally game-changing development that we've had," Hall added. "One of the things we've always been able to count on is that Dexter's secret is his own, and it's not anymore."

As a result, Season 7 will pose new questions, including whether Deb will turn her own brother in — and what he might do to stop her from doing so. "Dexter is a serial killer. The thought has to at least have crossed his mind at some point that that would be an easy solution to this problem," executive producer Scott Buck said of whether Dexter considers just killing his sister. "We don't shy away from the reality that there's no easy way to deal with this situation."

Dexter: Deb pieces it all together in new promo

While fans have been waiting patiently — and sometimes not so much — for Debra to finally catch on, Buck and Colleton said the time was ripe heading into the final two seasons for it to happen. "We knew we would always eventually tell the story, but because it so dramatically changes the nature of the story, we didn't want to consider it until we were closing in," Buck said. Added Colleton: "We thought, 'OK, this is the right time for this to happen.'"

Deb won't be the only woman Dexter will have to deal with this season. Not only will La Guerta (Lauren Velez) make a startling discovery in the premiere, but Chuck's Yvonne Strahovski will intersect with Dex beginning in Episode 3. "It's safe to say that Hannah McKay has a dark past that is slowly revealed as the episodes go on," Strahovski said. "She holds some secrets. She meets Dexter because she's helping the Miami police department with a case. It's a case that's been around for a while."

Still, Hall says the more problems Dexter has is exactly what will make the seventh season so enticing for viewers. "Dexter is never more compelling than when he's in trouble and he's never been in deeper trouble than he is in now."

Dexter returns Sunday, Sept. 30 at 9/8c on Showtime.