Bobby tells Dean and Sam that Castiel has stolen the Moishe Campbell Diary, but he has a copy. They read the journal and discover that in 1937, H.P. Lovecraft had opened a gate to Purgatory with five worshipers of the devil. Meanwhile Crowley abducts Lisa and Ben to force Dean and Sam to step back. Sam summons Balthazar for help and the angel is surprised by the revelations of the partnership of Castiel with Crowley. While the boys seek out Lisa and Ben, Bobby investigates what happened on 10 March 1937 and discovers the truth about his friend Ellie. Balthazar brings the brothers to an angel-proofed warehouse where Lisa and Ben are held captive. When Dean reaches them, he discovers that Lisa is possessed by a demon.
Crowley has Ben and Lisa kidnapped to get to Dean. Sam and Dean are forced to join with an unlikely ally to free them.
Directly following Castiel's visit in 6.20 The Man Who Would Be King, Bobby realizes that his copy of Moishe Campbell's journal is missing. Bobby tells Dean and Sam that he thinks Castiel stole the journal so that he could read Moisha's notes about H.P. Lovecraft, an author who was fascinated with the concept of doors to other dimensions. Thankfully, Bobby already made a copy of the journal, and as they discuss the possibility that Lovecraft knew something about Purgatory, Dean receives a call from Ben.
When Dean answers his phone, Ben tells him that he and Lisa are in danger. Men broke into their house and killed Matthew, Lisa's new boyfriend. Dean tells Ben to go out his window, but he's too late, and Ben is apprehended. The next thing Dean hears over the phone is Crowley's voice. Crowley offers an ultimatum: unless Dean and Sam back down and let them open the door to Purgatory, he'll kill Lisa and Ben. Dean refuses to back down, and, because they no longer trust Castiel, he and Sam summon Balthazar to ask for his help. They tell Balthazar that Castiel is working with Crowley, and that Crowley has taken two people that are important to them. Balthazar acts blasé, but they can tell that he's surprised by the news of Castiel's partnership with Crowley. He leaves before making any promises to help them with Lisa and Ben.
Meanwhile, Bobby questions a fanatic H.P. Lovecraft fan named Judah about the last weeks of Lovecraft's life. Apparently, Lovecraft held a dinner party on March 10, 1937 at which he and his guests attempted to open a door to another dimension. Castiel got to Judah before Bobby, however, and the information about the party Judah had is now missing. Bobby calls Sam to let him know about the ritual, and that, while all the original party guests died or went missing within a year of the party, he's going to question the maid's son, a man named Westborough, who was nine at the time and is now in a mental institution.
Without Balthazar's help, Dean and Sam capture and torture demons in the hopes that one of them will tell them where Lisa and Ben are. Dean takes the lead and, when he refuses to let Sam aid in the interrogations, Sam prays to Castiel for help. Castiel doesn't answer, but he hears what Sam says, and he goes to confront Crowley. He's upset that Crowley went behind his back to kidnap Lisa and Ben, but Crowley refuses to tell him where he's holding them. While they argue, Castiel receives a call from Balthazar and leaves. Balthazar confronts Castiel about his deal with Crowley. He doubts the viability of Castiel's plan to absorb half the souls in Purgatory - if Castiel can't handle the influx, he could explode and "take a substantial chunk of the planet" with him. Still, when Castiel asks him for his loyalty, Balthazar gives it.
When Bobby interviews Westborough, he learns that Castiel was again there before him. But Westborough didn't trust Castiel, and so didn't tell him everything about the night of the dinner party in 1937. Apparently, the dinner guests truly believed that nothing came out of the portal they created to Purgatory, but Westborough, because he claimed that something invisible came out of Purgatory and possessed his mother, has been in a mental institution ever since. He shows Bobby a picture of his mother, Eleanor, dated 1935, and Bobby exclaims over what he sees. Meanwhile, Dean gets sloppy interrogating a demon, and he escapes his bonds. He's strangling Dean when Castiel appears and kills the demon with his angelic touch. He tells Dean that he's always been there for him and that all he wants now is his trust, but Dean can't give it to him, and Castiel leaves. Bobby finds Eleanor Visyak in a cabin - one of her 'safe houses' - and tells her he knows who she really is. She confirms that she is a creature from Purgatory, but denies that she killed H.P. Lovecraft. She tells him that he's lucky that she's what came through the portal, because she's happy on Earth and doesn't want it destroyed. She's been trying to keep the door closed for the last 75 years, which is why she gave Dean the Sword of Brunswick to kill the dragons. Bobby warns her that Castiel is looking for her and that he wants to reopen the door to Purgatory. He asks for the information about how to open a door himself, but she refuses to tell him, and also rejects his offer of protection.
Back at Bobby's house, Balthazar appears and tells Sam and Dean that he's decided to be their double agent because Castiel is out of control. He's also found Lisa and Ben, but can't get close because Crowley has angel-proofed the building they're in. They seem reluctant to trust Balthazar, but he takes them to where Lisa and Ben are being held and then leaves. Sam and Dean enter the building and split up to search for Lisa and Ben. Sam is quickly jumped by two demons and locked, unconscious, in a room. Dean is able to fight his way through a number of demons to get to Lisa and Ben and free them, but as they move to leave, Lisa grabs Ruby's knife and holds Ben hostage. She has been possessed by a demon, one that tortures Dean and Ben with what she says are Lisa's thoughts about them. Dean tells Ben to ignore her hateful words and throws Holy Water on her.
Ben is able to get away and Dean disarms her, but when he starts to exorcise her, she grabs a straight razor from nearby and stabs herself in the stomach. Dean finishes the exorcism and she collapses, so Dean moves to help her. He tries to call Sam, but he doesn't answer, so Dean picks her up and gives his shotgun to Ben so that they can shoot their way out of the building. On their way out, they hear Sam banging on the door of the room where he's being held, and Dean shoots the lock to free him. They leave together and Sam hot wires a car so that they can speed to a hospital with Lisa. At the hospital, Dean and Ben sit at Lisa's bedside.
She is unconscious and when Dean tries to apologize for what happened, Ben leaves. Castiel appears immediately afterwards and, without Dean having to ask, heals Lisa. Dean tells him that it doesn't change anything, but he'd also like Castiel to do something else for him: wipe Lisa and Ben's memories so that they don't remember Dean. Once their memories are wiped, Dean approaches them and apologizes for hitting their car and putting them in the hospital. They accept his apology and he leaves. When he gets outside, Sam is waiting for him, and Sam is angry with Dean for messing with Lisa and Ben's minds. Dean tells him to never mention Lisa or Ben to him again (or he will break Sam's nose) and they drive away. As Eleanor Visyak leaves her cabin, Castiel appears and takes her.