Trial and Error is the fourteenth episode of Season 8 to air.
Kevin deciphers the tablet and figures out how to close the gates of Hell. He tells Sam and Dean that according to the tablet one person must face a series of three tests designed by God himself. The first one - kill a hellhound. Sam and Dean argue over who should complete the trials.
Kevin awakes on his boat, going through his day-in, day-out routine of eating hot dogs, drinking coffee, popping pills and deciphering the tablet, until many days' work finally leads him to a revelation, a nosebleed and a faint for his trouble. Elsewhere, Dean happily arranges his new bedroom within the "Men-of-Letters" complex, even reveling in new-found cooking skills to Sam's delight. As they eat, a distressed Kevin calls Dean and urges Sam and Dean to come quickly, sending the brothers rushing out.
Sam and Dean carefully enter Kevin's boat, finding the disheveled young man overjoyed about his revelation how to close the gates of Hell. Kevin reveals that in order to accomplish the feat, a spell will have to be performed after the accomplishment of three specific "trials" of God's design, the first of which will involve bathing in the blood of a hellhound. Dean heads out to secure necessary defenses, while Sam urges Kevin to take better care of himself and see light at the end of their ordeal.
Reasoning that hellhounds would most likely surround someone involved in a Crossroad Demon deal, Sam and Dean happen across a family in Shoshone, Idaho that struck oil ten years previously where no one thought possible, a good sign of a deal with a demon. The brothers travel to the Cassity farm, meeting the farm's young, beautiful manager Ellie, and posing as brothers applying for the job of farmhands. Along the way, the boys meet the Cassity family's "trophy husband" Carl Granville, who helps them secure the jobs.
Working in the stables, Sam and Dean eye oldest daughter Alice Cassity as most likely to have made the Crossroads deal, but while focusing on Alice, they fail to spot Carl being targeted and killed by a hellhound. As police cart away the body Dean suggests that they blew their chance, but Sam thinks there still might be more in play with the family.
Sam talks with Alice, deducing from her strangely vanished affection that Carl made the deal for her love years earlier, but not the family's inexplicable fortune. Sam theorizes that whatever Crossroads Demon may have made multiple deals a decade prior, and convinces Dean to stick around a few more days.
The next day, the rest of the Cassitys arrive, including its lingerie model-marrying patriarch Noah, faded country singer Cindy, and youngest daughter Margie, all of whom could potentially have made Crossroads deals. Ellie tasks Sam with waiting on the family for dinner, while Dean handles the grill for the night. Listening to them bicker amongst one another, Sam realizes that the demon they inadvertently dealt with a decade prior was actually none other than Crowley himself!
As Sam and Dean plot their next move, Kevin calls with newly uncovered information that glass scorched with holy fire will enable them to see hellhounds, to which Dean prepares eye-glasses for both brothers. Sam notices Noah and Margie headed out to the woods with rifles to hunt the beast that killed Carl, and offers to lend his expertise in assisting. Meanwhile, Dean tests out the glasses as Ellie makes a pass at him, which Dean regrettably turns down given the situation at hand.
Out in the woods, Sam follows what he believes to be sounds of the hellhound, but in his pursuit he fails to stop the real creature from attacking and killing Margie, to Noah's great horror. Sam and Dean gather the remaining Cassitys in their living room and explain the situation, sealing the room with goofer dust to keep out the hound, and handcuffing the family until they can identify who made a Crossroads deal.
Dean elects to hunt the creature while Sam protects the family, hopefully by killing the beast. Sam is determined to go with him, now that the family is locked down within the goofer dust. Dean has decided, however, that he should be the one to undergo the trials, and not Sam. Dean believes that the trails will bring death to one of them and that Sam is the more valuable of the two, the one who actually sees a light at the end of the tunnel, and the one to potentially have a normal life awaiting him after this quest. Dean leaves to undertake the battle alone, and warns an exasperated Sam that if he follows, he will put a bullet in his leg.
As the Cassitys continue bickering about which of them has fortunes that could only be explained by a sold soul, they finally deduce that it was Margie, who always said "if we were rich, we'd be happy."
Dean finds Ellie dancing alone in her room, and enjoying herself with a beer. Dean explains that an evil creature has been stalking the grounds, but Ellie surprisingly reveals that it has come for her, hence the attempted one-night-stand and drinking before she goes. Ellie explains that she made the deal with Crowley to save her mother from sickness knowing that she wouldn't go to heaven when she died, but that Crowley never said anything about the 10-year deal or about any hellhounds coming to retrieve her. Dean realizes that "douche-bag" Crowley also never warned Carl or Margie about the fine print of the deal.
Ellie sees Dean's face distort, signaling the proximity of the hellhound. Dean dons his glasses and stalks after the creature through the stable, but ends up slashed and flung across the room, breaking the glasses. Sam saves Dean at the last moment by shooting the creature, and drawing it toward himself, struggling with it before slitting its throat and dousing Sam in the beast's blood.
Setting up Ellie with a hex bag to protect her from further attacks in the interim, Dean tries in vain to perform the spell signaling his completion of the first trial, though Sam was the one marked by the hellhound's blood. Rather than find another hound, Sam elects to be the one to undertake the trials, not treating the quest as a suicide mission like Dean would. Sam assures Dean of his worth as a hunter, and expresses his intent that both of them live through the ordeal, and recites the incantation for the spell. Its effects cause him to double over in pain, as his right arm surges with bright light, before he recovers his composure.