Two Days Before Halloween
The town is preparing for Halloween and a mother returns home as her husband Luke Wallace feeds their baby child. The wife puts the candy away and takes the baby upstairs. Once she's gone, her husband gets into the candy and has a piece. Flinching in pain, he removes a razor blade from his mouth and then collapses to the floor, spitting up blood. His wife returns to find him dead on the floor.
One Day Before Halloween
Luke's wife explains to Dean and Sam that her husband swallowed four swallowed four razor blades. Dean checks out the stove and finds a hex bag underneath the counter. Sam asks if Luke had any enemies or was involved in an affair but she fervently denies it. Mrs. Wallace points out that there are better ways to kill someone than planting a razor in candy he might not eat. Back at the hotel, Sam determines that the bag contains powerful relics suggesting a skilled witch, while Dean is unable to come up with anybody who wanted Luke dead.
Two girls are attending an early Halloween party and one of them, Tracy, goes bobbing for apples. She comes up with an apple and the other one Jenny wants to try. She goes down… and can't get her head up. They try to pull her out and the water starts boiling. Dean and Sam arrive at the scene and Dean talks to the other girl, who doesn't know any Luke Wallace. Sam finds another hex bag and they take it back to the hotel where Sam figures it isn't revenge, it's a series of sacrifices culminating at Halloween. The blood sacrifice summons the demon Samhain, the origin of Halloween. The ritual can only be performed once every six hundred years, and once Samhain is raised he can raise ghosts and zombies and every dark thing they've ever faced.
Dean is watching Mrs. Wallace's house when Sam calls in. They figure there's some connection as someone had to deliver the hex bags. He watches as the babysitter arrives at the house: Tracy, who claimed not to know the Wallace. Dean checks in with Sam who discovers Tracy was suspended from school after a fight with a teacher. The brothers go to the school and speak with the art teacher, Don Harding. He says that Tracy's work had gotten increasingly disturbingly and wrote dozens of bizarre symbols and primitive killings. Sam shows him a symbol from one of the items in the hex bags. The teacher reveals she lives in an apartment alone as an emancipated teen.
The Winchesters try to find Tracy without success. As they return to the hotel, a kid asks them for candy. Dean refuses to give him any and the boy glares at him. They enter the room and find Castiel and another man waiting for them. Sam falls all over himself introducing himself to Castiel, who congratulates him on stopping his extracurricular activities. Castiel and the other man asked if they've stopped the raising of Samhain and killed the witch. Castiel warns the witch knows who they are and shows them a hex bag he found in the wall of the room. Castiel explains that the raising of Samhain is one of the 66 seals. The witch is powerful enough to cloak herself from angels, but the other man has had enough. Castiel explains that he's Uriel, a "specialist." Castiel says they both need to leave town as they're going to destroy it. They're out of time and the seal must be saved. Uriel explains he's purified cities before and they have to weigh the lives against the billions of others at risk. Castiel insists the plan comes from Heaven and it's just, but Dean and Sam aren't so sure. Castiel points out that Dean obeyed his father's orders. Dean tells the angels that they're not leaving, pointing out Castiel went to the trouble of breaking him out of Hell. Uriel threatens to drag him out but Dean warns that he'll have to kill him to do it. Dean insists they'll find the witch and Castiel agrees to give them the time over Uriel's objections.
The guys return outside to find that the kid egged the Impala. As they get in, Sam admits he thought the angels would be different: righteous. He wonders if this has been what he's praying to. Dean notes that it's possible not all angels might be good. Sam notices that one of the items, a child's bone, is charred with incredible heat, more than found in a fire or household stove. They go back to the school, figuring that the bone was charred in the art room's kiln. The hex bag appeared in their room after they talked to Harding. Sam breaks open a cabinet and finds more children's' bones.
Uriel and Castiel watch children trick-or-treating and Castiel notes that the decision has been made. Uriel suggests they get Dean out of town and destroy the place, but Castiel asks if Uriel's ready to disobey their true orders.
At the Wallace house, Harding is performing a ritual involving a bound and gagged Tracy. He prepares to stab her… and Dean and Sam arrive and Sam guns him down. Tracy reveals that Don was her brother and smashes them both down with her powers. She explains that Harding was going to make her the final sacrifice, but now she's going to use her brother. She takes some of Harding's blood and proceeds with the ritual. Sam manages to get to Harding's corpse and smears some of the blood on his face and Dean's. He tells Dean to follow his lead as the floor cracks open and a black demonic mist flows out and into Harding's body. It rises up, a host for Samhain, and the demon advances on Tracy. It takes her and kisses her, compliments her, then snaps her neck. Samhain walks by Dean and Sam without seeing them. Once he's gone, Sam explains that in Halloween lore people wore masks to hide from Samhain so he gambled the blood masks would conceal them.
As Samhain (still in Harding's body) walks among the trick-or-treaters, Sam figures he's heading for a cemetery to raise the dead. As they drive there, Sam says he might have to use his powers to stop it. Dean believes that Ruby's knife should be enough and gives it to Sam. At the cemetery, Samhain arrives at the mausoleum where a party is going on. He locks the students in and then the corpses start to emerge. They kill one student and Dean gets the others out while Sam goes after Samhain. Sam finds Samhain in the chapel, and its powers don't work on it. The two fight hand-to-hand while Dean impales zombies. A ghost arrives and throws Dean against the wall, and he decides to torch everyone. Sam tries to get to Ruby's knife but Samhain knocks it away. Sam finally tries to immobilize Samhain with his psychic abilities and expel the demon. Samhain advances step by step against Sam's powers as Dean arrives. Sam sees him as his nose starts to bleed, but he manages to expel Samhain once and for all.
One Day After Halloween
Later, Sam is packing up at the hotel when Uriel appears behind him. The angel notes that November 2nd is the anniversary of the days that Azazel killed Sam's mother and girlfriend. Uriel accuses him of using Azazel's profane blood despite the warnings. The angel notes that Sam will always remain alive as long as he's useful. He also advises Sam to tell Dean to climb off of his high horse, and to ask Dean what he remembers from Hell.
Dean is in the park when Castiel appears next to him and explains that their orders were to do whatever Dean told them to do. It was a test to see how Dean would perform under pressure. Dean says he'd have made the same call and what is here and now is what matters. Castiel confesses he was praying that Dean and Sam would save the town. However, he warns that the seal was broken and they're one step closer to Hell on Earth. The angel admits he has questions and doubts and doesn't know what is right or wrong anymore. Castiel doesn't know if Dean passed or fail, but warns that Dean will have more difficult decisions to make.