The Leviathan-possessed Castiel walks over to Dean, black veins spreading across his face. Bobby wakes up from where he was thrown while Dean points out that the Castiel vessel is about ready to explode as it leaks black fluid. The Leviathans promise they'll be back for Dean and then walk away. As they leave, they're unaware that Sam is down the hallway, apparently being choked by Lucifer. Lucifer insists that the last year has been one of his mind games with Sam, who is still in his cage in Hell. Dean and Bobby find Sam, who apparently snaps out of his hallucination of Lucifer, and they leave together.
Castiel heads to the nearby reservoir and wades into the water. The brothers and Bobby follow and watches as the water is sucked into Castiel and then a cloud of the black fluid spreads outward into the public water supply. Castiel's trenchcoat washes up on shore and they figure that he's dead.
The black fluid spreads across the area. A young girl, Annie, is possessed when she drinks from a fountain, and a water pipe explodes in a mechanic's face. The mechanic, Edgar, shakes briefly as his eyes turn black.
Later, Dean checks on Sam and treats his injured hand. Sam is the only one who can see Lucifer, mocking Dean. Bobby comes in and they figure that the Leviathans will soon reveal themselves. Dean wants to know if Sam is okay and points out that he's been lying to them. When he asks what happened, Sam explains that he's "seeing through the cracks" and having trouble figuring out what's real and what isn't. He insists that he didn't talk to them about it because they had enough going on, and wanted to try and read it out. However, Sam admits that it's getting more specific and tells them about Lucifer. Dean wonders why Lucifer would give him an illusion of a real life, and Lucifer tells Sam that he has to give him something so that he can take it away. When Sam glances over at Lucifer, Dean realizes that Sam is "seeing" him right now.
The possessed girl watches television in her bedroom and tunes into an ad for Dr. Sexy, M.D. A character sarcastically comments that Dr. Sexy can cut into anyone he feels like.
Bobby and Dean watch as Sam field-strips a weapon. Dean secretly turns on the GPS on his brother's phone and Bobby asks how he's doing. They wonder if there's something they can do for Sam, but Bobby asks Dean how he's doing. Dean insists that he's fine and Bobby points out that Dean was pissed when Sam said the same thing. Bobby isn't convinced and tells Dean that he'll be there when he wants to tell the truth.
At the high school, three members of the boys swim team enter the locker room and find two of their teammates waiting for them. As their eyes bleed black, the two boys say they heard something in the showers and then attack their former friends.
At the hospital, an older woman, Madeleine Harris, complains to Sheriff Jody Mills, who is in for an appendectomy, about how surgeons tend to lose their operating tools in their patients. Dr. Gaines comes in and assures Jody that she'll be fine.
Bobby picks up a report about the swim team attack and Sam says that Dean will have to check it out on their own. Dean isn't convinced that Sam is safe to leave behind but Bobby says that he can watch him.
The mechanic, Edgar, meets with the girl, Annie, and they discuss their respective bodies. The Edgar Leviathan says that his host body is skilled in demolitions, and then tells the Annie-Leviathan to get the ones occupying the swim team under control. Edgar warns that the boss wants it cleaned up and doesn't like bringing him bad news. Annie considers and then says she needs Edgar's help with an idea.
As Dr. Gaines goes by a hospital room, he finds Annie inside and asks if she needs help. She asks if he's Dr. Sexy and if surgeons can cut into bodies whenever they want. Annie then grabs his arm and says that she wants to be a surgeon when she grows up, and she wants to grow up right now. Annie takes on Dr. Gaines' shape and kills him.
Dean arrives at the high school locker room, posing as a FBI agent, and spots the black fluid on the wall.
At the hospital, Jody wakes up and apparently sees Dr. Gaines gassing her roommate unconscious and then taking her out for surgery. Once he leaves, Jody goes after him .
At the house, Sam gets a call from Dean saying that two of the swim kids are missing and the black fluid is all over the locker room. The two kids were picked up on a surveillance camera and Dean is heading back to pick up Bobby. As Sam sees Lucifer sitting in the room, he says that he's doing okay and hangs up.
Jody follows Dr. Gaines to a wing of the hospital under renovations and watches as he operates on her roommate. She staggers away and runs into a nurse, and then faints from the stress. Jody wakes up later and discovers that the nurse has put her back in her bed. Dr. Gaines comes in and asks what she was doing wandering the halls, and Jody insists that she didn't see anything. He jokingly threatens to go back in to fix any sutures if she keeps wandering the halls and the nurse gives her a sedative. Once they leave, Jody tries to get to her phone.
Sam asks Lucifer why he doesn't just end the dream, and Lucifer says that he has Sam right where he wants him. It will only end when Sam can't take it anymore, and points out that may be why Sam was cleaning his guns. Bobby comes in and finds Sam yelling at thin air. He offers Sam a beer and assures him that he beat the Devil before and he'll get a handle on his current situation. Sam hallucinates Lucifer ramming a poke through Bobby's chest and agrees with him. Jody finally manages to call Bobby and tells him that her surgeon is a monster. She knows that Bobby hunts monsters and tells him to get down there to help her. Bobby tells Sam to stay there and watch the phones while he goes to check out Jody's story.
Edgar and the two swim team boys arrive at the hospital and Dr. Gaines brings them in. He gives them a tour of the hospital and explains how it works, and how no one ever questions it when people die. He has the head nurse and staff administrator tied up. Edgar congratulates him and says that's exactly what the boss wants, and tells the two swim team boys to take over the bodies. When Gaines asks if Edgar is going to stay, he says he has to check out some other businesses in the area.
Sam is field-stripping guns again when Dean pulls up. He explains that he followed the two swimmers to Sioux Falls and that he needs Sam for backup because Bobby isn't available. Sam has no choice but to grab a gun and go with him.
At the hospital, Bobby gets Jody out to the taxi and sends her home. He then goes to the morgue and finds Mrs. Hackett's body. Much to his surprise, it's already been autopsied.
As they drive, Dean asks Sam how he's doing and suggests he get some help. Sam doesn't see the point since the doctors can't treat him. As they drive, dean warns that he'll never be okay.
The real Dean arrives at Bobby's and discovers that Sam is gone, and realizes what's happening.
Sam and "Dean" pull up outside an office building and see five workers inside. As the go inside, "Dean" says that Sam can't control himself. They go inside and "Dean" transforms into Lucifer, and Sam realizes that they're in a factory boiler room. He turns and shoots at Lucifer only to miss repeatedly. Lucifer tells him that if he wants to get it over with, he should shoot himself.
Bobby is examining Madeline's body when Dr. Gaines comes in. Gaines knows him and explains they saw him through Castiel's eyes. Bobby hits him and then grabs a shotgun and blasts him, but it has no effect. Gaines' mouth splits open revealing a huge mouth filled with teeth and Bobby runs for it.
Dean follows Sam's GPS to the factory and goes inside, calling for his brother. Lucifer transforms back into Dean, and Sam draws on Dean. He explains that he doesn't know how he got there or whether Dean is real. When Lucifer taunts him, Sam turns and shoots him and Dean tells him to put the weapon down. He reminds Sam that he's also been to Hell and that he knows that it feels different than reality. When Sam says that he can't possibly know, Dean asks to see his wounded hand. The wound is there and Dean says that he was there and bandaged the wound. He squeezes the wound and insists that the pain is different. Lucifer starts to flicker but insists that it doesn't mean anything. Sam digs his thumb into the wound while Dean says that they got him out of Hell and asks Sam to start by believing him and work from there. Sam manages to get control and his phone rings. It's Bobby, who says the hospital is their kind of thing and silver buckshot had no effect. He's heading for his house to regroup and Sam passes the message onto Dean.
As the brothers drive back, Sam insists that he's fine. As they pull up, they discover that Bobby's house has been burned up. They search the ruins but don't find any sign of their friend. Dean calls Bobby's cell phone and leaves a message admitting that he can't handle everything that's going on. As Sam searches, Edgar confronts him and admits that they're worth eliminating personally. Dean shoots him without affect and they struggle. Edgar hits Sam with a pipe and beaks Dean's leg, and Dean manages to drop a car on him. He's unable to wake his brother up and dials 911. The EMTs arrive and take them to Sioux Falls. When Dean learns they're going there, he begs the EMTs to take them somewhere else, but they don't take him seriously. Meanwhile, Sam sees Lucifer seated in the back of the ambulance, and the hallucination says that Sam hasn't gotten rid of him. Sam passes out as they near the hospital.
At the junkyard, the black liquid flows back into Edgar's crushed body.