With the “Supernatural” season 10 finale rapidly approaching, the Winchesters are racing against the clock to save Dean from the Mark of Cain. Star Jensen Ackles admits that while his remaining allies — Sam (Jared Padalecki) and Castiel (Misha Collins) — are determined to find a cure for the ancient spell, Dean “knows that there’s no way out” from Cain’s curse, which seems even more obvious now that one of his closest friends, Charlie (Felicia Day), has died in the attempt to heal him.

“Instead of forcing his support group to help a helpless cause, it just seems fruitless to him, so he basically says ‘guys, stop, leave it alone, there’s nothing you can do, and there’s nothing I can do except fight until I can’t fight anymore,'” Ackles recently told Variety. “And unfortunately, we’re gonna see what that means in the last episode.”

Unlike many of “Supernatural’s” previous seasons, the greatest threat to the Winchesters isn’t an external “big bad,” but “the bad within themselves,” according to Padalecki. “For Dean… the big bad is a seemingly inanimate object on his forearm.”

Ackles agreed, “This is a battle that is internal, and it’s not something that they can lock and load their shotguns and go after together. Unfortunately, Sam can’t find a cure, and of course, the loyal brother that Sam is will die trying, and Dean knows this. It really is this inner struggle where Dean can’t seem to get a grasp on the evil power that’s inside him, and Sam can’t seem to let go of trying to save his brother. We’ve seen similar things with this before with the demon blood and soulless Sam; it’s usually flip-flopped as far as the characters go, but it’s a heavy internal struggle that finally comes to a head at the end of the season.”

Collins told Variety that Team Free Will is going to “make deals with the devil and the ends justify the means and enormous sacrifices are made in our efforts to cure Dean.”

He wryly added, “There’s a moment in the last episode we shot — we’re shooting things out of order so I can’t remember which episode is which, but suffice to say, there’s an episode towards the end of the season when we make a sacrifice that I was thinking, ‘How are our characters okay with this? This is terrible, what we’re doing. This is disgusting.’ We’re making some serious compromises in order to rescue Dean.”

Despite those big compromises and high stakes (or perhaps because of them), Ackles described the season ten finale as particularly satisfying: “It’s a good one — in fact, in my opinion, I read the script and I’m like, ‘this is one of the best scripts I’ve read in years,’ so much so that I was like, ‘I’m gonna write [showrunner] Jeremy Carver an email and tell him how amazing this script is.'”

“Supernatural” airs Wednesdays at 9 p.m. on The CW. The season ten finale airs May 20.