CSI: Miami

From the outside, Horatio Caine's season-long manhunt for escaped convicts on CSI: Miami made for a nice plot engine to drive the season.

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But for co-executive producer Barry O'Brien, aspects of the journey have allowed Horatio (David Caruso) to do something far more important: He was finally able to deal with the murder of his wife, Marisol (Alana De La Garza). "This season was the season of closure with Horatio and Marisol, for sure," O'Brien says. "[He] put her killer away once and for all. He stopped him after a horrifying crime spree. ... I don't know that he's ever going to find true closure, but he's ready to sort of accept what comes next."

What comes next could be Natasha Henstridge. The Species star plays a mysterious role in Sunday's Season 9 finale (10/9c on CBS), which finds Horatio and Frank Tripp (Rex Linn) bringing the last remaining fugitive back to Miami on a transport plane. When the plane goes down and the convict escapes once more, Horatio and his team must find him before more lives are lost.

Although Henstridge's character, Renee Locklear, is originally suspected of aiding the convict, it's ultimately revealed that she's an undercover Secret Service agent. And as bullets fly, so do romantic sparks between Horatio and Renee, guaranteeing Renee's return in the Season 10 premiere — and, most likely, beyond.

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"We set up the possibility for [sparks] to happen, but I give the actors all the credit for the way the relationship turned," O'Brien says. "It was David and Natasha. They look great in a two shot. They really reacted well with each other and it was all very natural and organic, so we all looked at each other and said, 'OK, more of this!'"

But even if Horatio has moved on from avenging Marisol, the manhunt has served another purpose this season. "I think something bigger may be going on with the character," O'Brien says. "If you look around, it seems that lawlessness is everywhere in the headlines. You look at what's going on around the world, and you see some crimes in our country becoming more horrific. I think characters like Horatio Caine are saying, 'No, not here; not on my watch.' ... He is willing to go deeper, in more dangerous places than ever before to sort of be the last line of defense in this lawlessness."

But how far will he have to go? The closing moments of Season 9 will put Horatio's life in the balance. "If you look back at the scenes in our season opener, he risks everything to save the entire team," O'Brien says. "In the season finale he puts his very survival on the line to save one. I think that's where the shock will come from.  It's totally unexpected of him."

CSI: Miami airs Sundays at 10/9c on CBS.