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.
Dean struggles with the after effects of the Mark of Cain, while Sam hears about a case where straight-laced people are turning into violent murderers.
Dean is frustrated when he can’t reach Crowley, who has promised to find The First Blade, but finds out soon that Crowley fell off the wagon and needs his help.
A teenage girl is killed and the final selfie she took before the attack shows a ghostly figure in the background, which brings Dean and Sam to Washington.
Sam and Dean Winchester are stunned to discover the Men of Letters' bunker is haunted by a very familiar ghost.
Sam and Dean investigate recent murders where the victims were hundreds of pounds lighter after their deaths, which leads to the two going undercover at a fancy day spa.
After finding out that Garth is in the hospital, Sam and Dean go to check on him.
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.
Dean's best chance of saving Sam lies with Crowley. Devastated after Kevin's death, Dean vows to find the angel that killed him, not only to free Sam, but to seek revenge. Castiel arrives at the bunker and suggests they try to separate Sam's brain from the angel's so they can talk to Sam and have him expel Gadreel. However, there's one small problem: the only person who knows how to do that is Crowley.
The Angel War begins! After a massive angel slaughter occurs, Sam and Dean arrive at the scene to investigate and are surprised to run into Castiel. While investigating, Castiel is captured and tortured. Meanwhile, Dean questions Ezekiel's demands and wonders if it might be time to tell Sam the truth.
Sheriff Mills (recurring guest star KIM RHODES) calls Sam and Dean for help investigating multiple murders in her small town. Discovering all four victims belonged to the same church chastity group, Sam and Dean decide to infiltrate by joining the group themselves. When Dean disappears, Sam and Sheriff Mills realize whatever they are hunting has taken Dean too . . . and they team up to rescue him.
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