Crowley realizes Dean’s demonic antics are starting to cause problems for him in Hell so he searches out an unlikely ally – Castiel.
Crowley notices Dean is becoming more and more aggressive. Dean tells him that’s the side effect of being a demon, but Crowley knows it’s something more.
Dean is a demon and running amuck with Crowley while Sam tries to figure out what happened to his brother. Meanwhile, Castiel is dealing with his diminishing grace.
In the shocking ninth season finale, Dean feels the effects of the First Blade, and Metatron makes his move against humanity. Meanwhile, Sam, Dean and Castiel face shocking consequences when taking the fight to Metatron.
After a massive attack on the angels, Castiel calls Sam and Dean for help. As they leave, Dean's eagerness to bring the First Blade doesn't go unnoticed by Sam who is worried about the cost to his brother whenever he uses the Blade. Meanwhile, Castiel is shocked when he learns the angel that caused the attack was one of his followers and did it in his name. Dean discovers there is a conspiracy amongst Castiel's angel followers and at the heart of it is Tessa, the Reaper (guest star LINDSEY McKEON)
Castiel captures one of Metatron's angels (guest star GORDON WOOLVETT) and asks Sam and Dean for help with the interrogation. Dean eagerly accepts, which doesn't go unnoticed by Sam. Meanwhile, Abaddon demands Crowley help her kill Sam and Dean. When he refuses, she reveals her shocking bargaining chip. Also, Castiel sets a meeting with Gadreel.
Sam and Dean investigate a case in Chicago, where they discover that various mafia-esque monster families are, unknown to humans, running Chicago's criminal underworld. One is a family of shapeshifters run by Margo Lassiter (guest stars DANIELLE SAVRE), whose leadership is thrown into question when her brother David (guest star NATHANIEL BUZOLIC) returns home. The other reigning family in town is made up of werewolves, lead by Julian Duval (guest star SEAN FARIS). While in Chicago, Sam and Dean meet Ennis (guest star LUCIEN LAVISCOUNT), a man with a personal vendetta against the monsters. The Winchesters warn Ennis about heading into the hunter lifestyle, but Ennis refuses to listen and starts down a dangerous path.
Sheriff Mills (KIM RHODES) calls Sam and Dean after she kills a vampire who attacked a prisoner named Alex (guest star KATHERINE RAMDEEN). They discover Alex was kidnapped by a family of vampires who use her as bait to lure in humans they can then feed on. Sheriff Mills tries to rescue Alex from her captors, but finds out the hard way that family always comes first.
The angel scribe Metatron, who's had exclusive reign over Heaven, attempts to get Castiel to join forces with him. Still furious with Metatron, Castiel refuses, setting a surprising plan in motion. Meanwhile, Sam and Dean capture Gadreel.
Series star Misha Collins makes his directing debut. While Dean struggles with the aftereffects of the Mark of Cain, Sam hears about a case where straight-laced people are turning into violent murderers. Sam suspects possession and suggests to Dean that they investigate, but Dean tells him to go without him. While interviewing the local townsfolk, Sam meets an elderly woman named Julia (guest star JENNY O'HARA), who tells him the Men of Letters came to town in 1958. Julia tells Sam the story of a young man named Henry Winchester (guest star GIL McKINNEY) and his female companion, Josie Sands (guest star ALAINA HUFFMAN). While Sam is away, Crowley tests Dean.
Dean is frustrated when he can't reach Crowley, who has promised to find the First Blade. When Crowley finally calls Sam and Dean, he tells them he fell off the wagon and has been injecting human blood and needs their help. To make matters worse, he had a demon (guest star Rebecca Marshall) helping him find blood, but she sold him out and told Abaddon that he and the Winchesters were searching for the First Blade. The brothers need to find the blade before Abaddon does, so they enlist the help of a former member of the Men of Letters (guest star Kavan Smith). Meanwhile, Nicole "Snooki" Polizzi appears in a cameo role.
When a teenage girl (guest star ALI WEBB) is killed, the final selfie she took before the attack shows a ghostly figure in the background. Looking for something to keep them busy, Dean and Sam head to Washington to find the ghost. While the two interview the girl's mother, Betty (guest star MARIA MARLOW), they are surprised when she starts talking about cold spots and fritzing. When they ask her about bringing up paranormal signs, Betty replies that the "supernaturalists" who called earlier brought it up. Enter Harry Spangler (guest star TRAVIS WESTER) and Ed Zeddmore (guest star A.J. BUCKLEY), the famous supernaturalist Ghostfacers!
Sam and Dean are stunned to discover that the Men of Letters' bunker is haunted. Their investigation leads them to a storage facility where the demons have been keeping a prized possession. Meanwhile, Castiel is captured by Bartholomew (guest star ADAM HARRINGTON).
Sam and Dean investigate recent murders where the victims were hundreds of pounds lighter after their deaths. Suspecting witches, the Winchesters go undercover at a fancy day spa that all of the victims had attended for weight loss. While Sam is hired as a fitness instructor, Dean takes kitchen duty. After Dean swipes some tainted food and passes out, the guys realize someone is drugging the guests and literally sucking all of the fat out of them until they die.
After finding out that Garth (guest star DJ QUALLS) is in the hospital, Sam and Dean go to check on him. Still carrying the guilt over Kevin, Dean lays into Garth about disappearing and demands to know where he's been. Garth makes an excuse and then hightails it away from the guys. Confused as to why Garth would run from them, Sam and Dean do some investigating and are stunned by what they find.
Looking to reclaim his place as King of Hell, Crowley asks Dean for help finding "The First Blade," the only blade that can kill Abbadon. Knowing he owes Crowley a favor for helping Sam, and also wanting Abaddon dead himself, Dean reluctantly agrees and the two set off on a road trip. However, Dean and Crowley are completely unprepared when they find out who has the blade – Cain (guest star TIM OMUNDSON), the first son. Meanwhile, Sam is back at the bunker with Castiel who notices there is some leftover grace inside Sam. If they can extract it, they can use the grace to track Gadreel. The only problem is the procedure may do permanent damage to Sam.
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