Sam and Dean investigate a bizarre murder in a college town. Their probe leads them to an apartment where they find two dead bodies and a computer with some disturbing footage that indicates the deaths may be the result of an unusual animal attack.
Castiel tells Sam and Dean that he wants to become a hunter, and he helps them investigate a case in which a man's heart literally burst through his chest. The probe reveals that other strange murders have occurred in town, all of which resemble deaths from cartoons.
Sam and Dean try to close the gates of hell, after Kevin deciphers the tablet and learns that one person must complete three tests designed by God to achieve the task. The first mission is to kill a hellhound.
Sam and Dean learn from Castiel that Crowley has unleashed several demons in a small town to search for Lucifer's crypt, which contains a very valuable asset. The brothers also discover that Crowley is torturing Meg because she knows the exact location of the crypt.
Sam and Dean investigate a series of vampire kills and discover that some orphans, who lost their parents to vampires, are being trained as hunters by a mysterious man. He claims he is helping the kids get revenge, but Dean is suspicious of his motives.
Sam and Dean receive a distressing video message from Kevin Tran, which prompts an attempt to uncover the Third Trial. Meanwhile, Crowley goes on a winning streak; and Castiel tries to elude Naomi and the angels who are hunting him.
The eighth season ends with Crowley poised to undo all of the good that Sam and Dean have accomplished as hunters. But the Winchester brothers try one final play against the Demon King. Meanwhile, Metatron gets Castiel to help him take on Naomi and the bureaucracy of heaven.