Jared Padalecki, Jensen Ackles Jared Padalecki, Jensen Ackles

Supernatural is not messing around.

On Tuesday's finale (9/8c, the CW), the Winchesters and Castiel (Misha Collins) finally face-off against Metatron in a battle that is poised to change everything. "The last five minutes of the finale are the most important five minutes of Supernatural history," Jared Padalecki tells TVGuide.com. "Something happens that has never happened before in the show. We've gone left, right, up, down, done all sorts of things in the show, but we do something in the finale that we've never seen."

"I haven't seen Season 10, but I'm imagining it has to set up all of season 10," the actor adds. "It's very exciting."

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With the Mark of Cain continuing to take its toll on Dean (Jensen Ackles), the eldest Winchester has been struggling against his darker instincts. But it seems Dean can't resist any longer. "We're going to see him go all the way," Padalecki says. "There's a parallel with Dean in Season 9 kind of going through what Sam went through in Season 4, where he feels empowered, he feels strong, he feels like he's doing the right thing. And obviously it can take over."

Of course, in Season 4 it was Dean who was able to get through to Sam (though not in time to stop him from breaking the final seal). Will Sam try to repay the favor and save his brother from himself?

"Yes he will," Padalecki says. "Sam did a lot of things this season that upset Jared — like, I wanted to be there. As a little brother and a father of two boys, I wanted to see Sam fight for his brother a little bit more. That didn't happen earlier for reasons understood, but we certainly see Sam in the finale be the brother that I would hope for him to be."

Having Sam be the one trying to help Dean is a nice change of pace, since Dean has always been the one (over)protecting Sammy. Last week, the eldest Winchester even went so far as to declare their relationship a dictatorship, which seriously goes against Supernatural's message of brotherly love. But never fear: According to Padalecki, the pair will put their past behind them and once again unite.

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"We see it in a strange situation, but we do see Dean understand that it's not a dictatorship, it's a partnership," Padalecki says. And while Dean constantly undermines his brother, Sam will finally prove himself in his Dean's eyes. "I think Dean will see that Sam loves his brother. Dean will certainly see that Sam is there for him and know Sam's commitment," Padalecki says.

But Supernatural isn't just about the Winchesters. Crowley (Mark Sheppard) and Castiel will also factor into the finale in major ways. While the newly reinstated King of Hell "plays a very integral part" in the game-changing final minutes, Castiel will be tested in ways the angel has never imagined.

"Something's going to have to give," Padalecki says of Cas, whose grace is quickly fading. "He's either going to have to figure out a way to get a different grace, and he's probably going to have to do it in the not most honest of ways. So, we get to see Cas be tested like human beings are." (Fortunately, fans can't be too worried about Cas, since Collins will be returning next year as a series regular.)

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As to the future of the series, which celebrates its 10th season next fall, Padalecki says he's more than willing to keep cruising in the Impala for a few more seasons. "I know Jensen is too. Hopefully Misha, as well. Hopefully the writers are," Padalecki says. "I feel like in Season 9 the quality of our show improved from the earlier episodes. We hit a new groove. We found characters and story lines that worked. So, if we can continue to find these story lines that work then absolutely. I would love to carry on."

Any theories on the finale's big twist? Do you hope Supernatural continues beyond Season 10?

Supernatural's finale airs Tuesday at 9/8c on The CW. Catch up on episodes here.

(Full Disclosure: TVGuide.com is owned by CBS, one of The CW's parent companies.)