Sam (Jared Padalecki) and Dean (Jensen Ackles) discover that hunters are being eliminated by their own kind in episode 21 of "Supernatural" season 12.

In the next episode of "Supernatural" season 12, titled "There's Something About Mary," the Winchester brothers find out that hunters are being killed by mysterious "accidents" in different parts of the country, the synopsis states. However, the brothers make a conclusion about the case that will make them worry about Mary (Samantha Smith).

As seen from the episode 21 promo, Sam tells Dean while examining a hunter's dead corpse in the morgue, "People who do what we do, hunters."

It appears that the people responsible for killing those hunters are also hunters themselves, which leads them to a hunch that the British Men of Letters might be behind these deaths. This makes Sam and Dean urgently search for their mother, especially after they received a message from her alerting them of a problem with the British Men of Letters.

Little do they know, Mary is already held hostage at the British Men of Letters headquarters and might have already suffered under the torturing methods of Toni Bevell (Elizabeth Blackmore).

Meanwhile, Sam and Dean have reason to believe that Cas (Misha Collins) has gone evil after admitting that he sees hope in the unborn child of the devil.

Previously on episode 20, titled "Twigs & Twine & Tasha Banes," the Winchester brothers discuss Cas' current stand in confusion as Dean tells his brother, "Cas said that he had faith in Lucifer junior? What the hell does that supposed to mean?"

Even though Sam tries to reason with Dean, the older Winchester claims that he did not recognize Cas from the events of the previous night and that he is convinced that Cas has changed.

"Supernatural" season 12 airs every Thursday night at 8 p.m. EDT on The CW.