Austin Nichols, Sophia Bush

While One Tree Hill may be known for soapy plotlines mixed in with a few psycho killers (RIP, Nanny Carrie), the heart of the series has always been the relationships. As the show embarks on its ninth and final season, we were on the set with the cast as they warned that it's definitely not all rainbows and butterflies for the core couples. From Nathan's absence to an accident involving one of Brooke and Julian's babies, see what to expect when One Tree Hill returns (Wednesday, 8/7c, The CW).

Nathan and Haley: When the show picks up, Haley is back to being a working mom now that she and Brooke have reopened Karen's Café. But as Bethany Joy Galeotti tells us, things quickly unravel from her "very glam, small-town entrepreneurial mom life."

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"Nathan is away, so that's taking a huge toll on their relationship," Galeotti says. Galeotti explains that she wonders who she is without her husband and how to keep him in her life while he's gone.

As the preview shows, Nathan's absence proves to be serious, but the actress assures us she's content with how Haley's journey ends. "I'm happy with Haley, what she's been through and where she's come. After seeing her whole journey, to have been there through everything and know that home is still home, I think it's a good ending."

Brooke and Julian: Brooke has certainly had it rough — romantically and physically (barely escaping death when she was attacked three seasons ago). But she finally met and married her true love, Julian. Although the couple is solid as ever, Sophia Bush says they're rattled to the core by an accident involving one of their twin newborns. "Sometimes it's just one little cog that gets off in the wheel and your life falls apart," she says. Austin Nichols, who plays Julian, adds: "[The] accident could happen with any parent. Julian puts a lot of blame on himself for this."

Even though it's hard to imagine putting Brooke through another tragedy, Bush says if her character was coasting, it would be boring and somewhat unrealistic. "No matter how together you are, no matter how professional you are, and no matter how many good things happen, we've all been through things that are almost unspeakable," she says. "It's important for me to have played someone who is flawed and who has been hurt and who has been betrayed, even though she's a great person."

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Clay and Quinn: As Rob Buckley, who plays Clay, told us: "Have we ever seen Clay not have issues?" So the couple is about to hit their roughest patch yet. "You're going to get a good chance to see if we got what it takes to go the distance," Buckley says. Shantel vanSanten, who plays Quinn, adds: "This season really tests Clay and [Quinn's] relationship and not in a way that we're in comas or it's life-threatening, but it really tests the strength of our love for another and the foundation that we started our relationship from."

If Clay and Quinn do outlast their impending turmoil, vanSanten says she doesn't necessarily believe a wedding is in the offing. "I don't know if I want us to get married. I want us to be quirky and unconventional ... and I think that if we make it through, there is a place where forever exists for them."

Chase, Alex ... and Chris Keller: With Chase back from Air Force basic training, he and Alex are rekindling their long-distance romance, until one of them gets cold feet. "They're both about to take some big steps in their lives ... and then maybe someone being a little reluctant," Stephen Colletti says. "Alex will go away for a little while ... so you'll see Chase having to deal with the absence of Alex." But, luckily, there's one singleton in town for him to hang with — Chris Keller, who returns to One Tree Hill after a long absence. "We totally bromance out," Tyler Hilton jokes about the friendship between Chris and Chase. "He never has guy friends and he just thinks Chase is the coolest, which is hilarious ... but he's still ornery as all hell."

Mouth and Millicent: Perhaps the cutest, most endearing couple in Tree Hill, they'll be hit with some major hardships this season. "Mouth has undergone a physical transformation," Lee Norris tells us. "[He's] clearly going through something and it's going to affect [Millicent] so I'd say you'll see a bit of both ups and downs." But Norris did make sure to tell us that although he can't say which pair has this season's wedding, "Mouth and Millicent will both find happiness."

And what about the couple's new morning show? Still going strong in the final episode. "I was filming [Mouth's last] scene, a broadcast with their TV show, and after I got a little choked-up," Norris says. "I thought this ties the ribbon for me on the whole experience."

Watch the first two minutes of Wednesday's premiere: