Ep 21. When the Levee Breaks

In the Supernatural season 4 episode 21, titled "When the Levee Breaks," the Winchester brothers are at odds with each other as they try to save Dean from succumbing to the demonic influence of the Mark of Cain. Sam is desperate to cure Dean of the mark's power, but Dean is insistent that he needs the power to protect others from the demonic forces that threaten them. This tension leads to a heated confrontation between the brothers that puts their already strained relationship to the test.

Meanwhile, Bobby and Castiel try to come up with a plan to stop the release of Lucifer from his prison in Hell, but they're quickly running out of time. The demons are gearing up for a full-scale attack on the Earth, and the Winchesters and their allies must act quickly to prevent the apocalypse from happening.

As the tension between Sam and Dean continues to rise, Sam begins to grapple with his own personal demons. He's haunted by the memories of his time with Ruby and is struggling to come to terms with the fact that he was manipulated by her for her own demonic purposes. The weight of his guilt is compounded by the fact that he was the one who convinced Dean to take on the Mark of Cain in the first place.

The episode builds to a climactic showdown as Sam and Dean's disagreements come to a head. With the fate of the world hanging in the balance, the Winchesters must find a way to reconcile their differences and work together to save humanity from being engulfed by darkness.

Overall, "When the Levee Breaks" is a tense and emotionally charged episode that sets the stage for the season's epic finale. As always, the writing is top-notch, and the performances from Jensen Ackles and Jared Padalecki are among the best of the season. Fans of the series won't want to miss this one!