The film opens with images of cave drawings, paintings on ancient Greek urns, Egyptian hieroglyphs, and medieval tapestries and paintings depicting humans either fighting monsters or being sacrificed to them -- establishing in the opening credits that, since the dawn of human history, every culture around the planet has espoused this belief that human sacrifice placates evil.
Cut to a scene in an office complex where Hadley and Sitterson (Bradley Whitford and Richard Jenkins) are talking -- they are clearly scientists/bureaucrats working somewhere that's classified and affiliated with the government. Lin (Amy Acker) is from the Chemistry Division and intrudes on their conversation as Hadley and Sitterson are about to begin a big project -- we learn there's competition not only amongst the divisions in this one office, but also outside competition because similar projects are underway in Sweden, Spain, Japan, and other countries.
The complex is deep underground and employs multiple divisions to run a "Truman Show" type of facility that's hidden in a remote area of what appears to be a midwestern state (possibly it's the Upper Peninsula in Michigan). The purpose of this facility is to fulfill the terms of an ancient treaty with powerful, sleeping, evil gods who demand a specific human blood sacrifice every so often in order to remain dormant. This has been going on since the dawn of time, only by the year 2012 humans have perfected it in such a way that science and technology have allowed the terms of the treaty to be fulfilled using the "Truman Show" type of controlled environment. The giant facility has the setting of a cabin in isolated woods near a lake. The cabin has a cellar that's stocked with hundreds of different occult objects; each object summons a different kind of monster. The scientists send unwitting teenagers into the facility in groups of at least five and then the teenagers stumble upon the cellar and interact with the objects...and each time this scenario is run, a different monster is summoned, and the teenagers die a different way with the scientists doing things to help the monsters win and ensure the teens die in a specific order according to the terms with the ancient gods. The group of five must always include a Whore, an Athlete, a Scholar, a Fool, and a Virgin. The Virgin is the only one who is allowed to survive, but she must suffer first at the hands of the monster. All the other teens must die, and die in the correct order. If the Virgin dies, the ritual fails. If more than the Virgin survives, the ritual fails. That's why there are teams running similar operations all around the world -- so that at least one group succeeds and the ancient pact is maintained. To date, only the US and Japan have been successful in fulfilling the terms of the ritual and it's a running competition between the two to see who can fulfill it fastest and most spectacularly.
This time around, the scientists have arranged for athlete Curt (Chris Hemsworth) to bring his four friends up to the cabin facility. The ruse is that a cousin Curt has never heard of before has offered him the use of a cabin he just bought near a lake. Curt asks his promiscuous girlfriend Jules (Anna Hutchison), her virginal friend Dana (Kristen Connolly), and their friends Marty (Fran Kranz) the stoner and Holden (Jesse Williams) the scholar to come along. The first 15 minutes or so of the movie are devoted to the characters proving they're the archetypes needed -- doing stereotypical things that fit their roles (like the jock throwing the football around, the stoner using a giant bong that collapses down to look like a regular travel coffee mug, and the promiscuous girl going on about her hair and the new dye she used to become even blonder). After a little back and forth and some wordplay, they pile into a Winnebago and are on their way to the cabin.
Stopping for gas at a creepy filling station on the abandoned road, the Scooby Gang (seeing as how the archetypes are almost exactly like the Scooby Doo characters, it's a good nickname for the group) encounter a creepy gas station attendant who warns them that "the old Buckner place" is someplace evil and they shouldn't go there. The gang mock him and leave after getting gas, not heeding his warning. Later in the film, it's revealed this man's name is Mordecai and he's filling the role of "the Harbinger," which is a requirement of the sacrifice. It seems the teens must be warned specifically that there's a risk of death...and they must willingly choose to proceed ahead anyway despite that known risk. It appears that "Morty" has been filling the "Harbinger" role for many years.
Once the teens get to the cabin, they explore around and encounter a great many creepy things -- such as a mounted wolf's head on the wall and a creepy painting depicting werewolves eating a deer that's covering a one-way mirror that's normally found in police interrogation rooms. Holden discovers the mirror and has a chance to spy on Dana with it, but he decides to be a gentleman and let her know it's there before she undresses. The Scoobies then remark on all the weirdness before settling in to drink from their keg and play truth or dare. Jules is dared to french kiss the wolf's head, and she does -- very graphically too. It's revealed that Lin the scientist put something in her hair dye that's making her act more trampy than usual...and there are pheromones and other chemicals being pumped into the cabin to make the teens act more sexually charged and aggressive than usual. This is to explain how in every horror movie the characters seemingly start doing the opposite of what normal people would do in the situation; Cabin in the Woods offers that they're acting this way because the scientists running the show have been drugging them to act this stupid.
The only one who is not affected is the stoner, Marty...and we learn later that's because whatever blend of drugs he's smoking in his bong has been interfering with the chemicals that the scientists have been using on him.
Marty keeps having the feeling that something not right and that they're all being manipulated by "puppet masters," as he calls them.
Suddenly, the cellar door is popped open, and Curt says "it must have been the wind." Dana is dared to go down into the cellar. She does -- and she discovers all the occult objects that can conjure monsters. Dana handles a dusty old journal (which can summon a zombie family into torturing people Texas Chainsaw style); Jules plays with a cursed necklace on an old wedding dress (which can summon poltergeists); Curt holds a conch shell and almost blows into it (if he had, then he'd have summoned Mermen from the lake who would have killed them); Holden is captivated by a music box with a ballerina in it (which would have summoned Sugar Plum Fairies with faces made of teeth); Marty looks at some old movie film (which would have brought werewolves to life); and Curt also plays with an orb that's like an old puzzle box similar to the Hellraiser movies (and it would have summoned a PinHead-like demon to torture them).
This is where the teens "choose" what monster will come kill them...and there are limitless possibilities (as we'll see in just a few paragraphs here).
Dana wins out by reading from the journal and then reciting a Latin spell that brings the Buckner family back to life in the woods.
In the government facility, the scientists had a pool running to see what monsters would have been picked. There was a giant board with all sorts of choices that will be fun to look at on the DVD because it goes by too fast to read all the options that would have been available. Hadley had been rooting for the Mermen as apparently they are never used (despite Curt getting so close to blowing on the conch shell). The Maintenance Department won the pool because every year it picks the Buckners. Tom Lenk from Buffy the Vampire Slayer makes a cameo as an intern who also picks the Buckners and splits the betting pool with them, as a result.
Curt and Jules at this point have the subliminal thought put into their heads by way of whispering that only Marty can hear, telling them to go outside to have sex in the woods. Marty wonders where the voices are coming from but the others tell him that he's nuts and say they can't hear anything.
Curt and Jules go outside, but Jules says she's too cold to get naked. So the scientists then raise the temperature in the 'Truman Show" facility and also pump in sex pheromones to make Jules and Curt act more stupid and want sex even more. The scientists even create mood lighting by making the moon shine on this one patch of grass where it's perfect for them to have sex.
Just after Jules takes off her top and shows her bare breasts to Curt, the zombies attack; these zombies can use tools so they are different than regular zombies. Instead of just wanting to bite people, these zombies want to cut them up and torture them. Jules is quickly killed and decapitated while Curt makes it back to the cabin...just in time to save Marty from being killed by a little girl zombie (the writer of the journal that Dana read).
Inside, the group at first decides to stick together to barricade themselves in the house...but the scientists whisper that they should split up, and so they do (just like in horror movies). This of course, makes them easier to kill. Meanwhile, down in the facility, the scientists turn a lever and make blood flow into a carving of the Whore archetype...noting that the first victim has been offered to the sleeping gods. On the screens in the control room, we learn that all the other countries have failed in their efforts except for the US and Japan...where in Kyoto the Japanese are using a ghost demon like in The Ring to torture school girls in a classroom (but there are no fatalities yet).
The Scoobies end up locked in different rooms...and Marty finds a camera and wiring hidden in the room, so he knows he's been watched the whole time. He realizes he is not imagining things and that there really are "puppet masters" at work.
Marty didn't barricade his window and a zombie bursts through and grabs him. They fight outside, with Marty using his bong as a weapon, but the zombie chops him in the back with a knife and seemingly carries him away to die. The scientists then pour blood on the second archetype engraving, showing that the Fool was killed. This leaves just the Athlete and the Scholar to die for the ritual to be complete -- with the Virgin having to either die last or survive for the ritual to be a success (which leaves us to assume that in the other countries besides the US and Japan, the Virgin died before she was the last one to survive).
The Scoobies then decide to make a run for it after discovering they could get into the cellar through holes in the floors. They all meet up under the cabin as the zombies keep trying to attack. There's no way to kill the zombies as they just keep coming no matter what.
The teens get outside and into the Winnebago but fail to notice that there is blood and goo on the door handle --- meaning one zombie is hiding inside waiting for them (just like in a horror movie). They drive the truck back down the road and are on their way out when the scientists realize that no one collapsed the tunnel -- this was a pass in the mountain that the teens drove through to get into the giant dome of the facility. The dome is concealed via technology to look like it's the forest and night sky when in reality it's a giant wall (that birds fly into and get zapped and killed). As the kids are racing towards the tunnel, the scientists realize there's a wire that's burnt out, and they try to fix that so they can destroy the tunnel and stop the kids from escaping. This is to explain how in horror movies the kids can never get away, and the monsters always seem to be getting some kind of unseen "lucky" help like this in order to kill them.
The tunnel is destroyed, and the kids have to back up the Winnebago. Curt then decides to take the motor bike from the back of the camper and jump the ravine to go get help. He says he's jumped deeper and longer cliffs than that before. Of course, when he leaps into the air and sails across the canyon he runs smack into the wall and is electrified/crushed and falls down to the ground. Dana and Holden realize that something is up and that not everything is as it seems, but they don't know what to do. Dana realizes they are trapped. Holden suggests driving deep into the woods and trying to make it out on foot, but Dana seems to know that the facility will be totally enclosed and that tunnel was probably the only way in or out.
Of course, the zombie that was hiding in the camper strikes...killing Holden while he was driving near a lake...and the camper plunges into the lake with the zombie almost killing Dana too...but Dana made it to the surface of the impossibly deep lake as the camper sank below the water.
The scientists all cheer because they think they have fulfilled the ritual since they think Dana is the only one to survive. Everyone is relieved because it seems Japan failed completely -- the school girls sang a song that destroyed the ghost demon and turned it into a happy frog, so the Japan scenario had no fatalities. If the US scenario doesn't fulfill the ritual, then the ancient gods will rise and destroy the world.
While the scientists are cheering, one of the zombies sneaks up on Dana while she's catching her breath on the dock in the lake. There's a red phone that rings from "Upstairs" and all the scientists are afraid because "Upstairs" never calls them. It turns out that Marty is still alive...meaning the scenario will fail if the zombie kills Dana and leaves Marty alive. Everyone panics as there's no way to stop the zombie from killing Dana.
Just then, Marty appears and knocks the zombie in the water. He and Dana then run back to the cabin, but instead of going inside, Marty leads Dana to the graves that the zombies crawled out of. He found a trap door in the graves that leads to a high tech tunnel underneath. He's figured out that someone brought those zombies into the tunnel via an elevator in the ground...and he's found a way to hotwire the elevator to go back down. There's a dismembered zombie on the floor that startles Dana (it's the zombie that everyone thought killed Marty, but Marty killed the zombie instead). Marty and Dana then descend in the elevator and are now in the facility.
If you watched Buffy the Vampire Slayer, this is similar to "the Initiative" labs underneath the town of Sunnydale...since the people who made this movie were behind Buffy.
The elevator doesn't just go up and down, but can go sideways too...and as it moves we can see more monsters through the glass as it passes other elevators. This is to show all the different variations that could have happened if the kids had picked something other than the zombie-summoning journal. There's a werewolf, ghosts, the Hellraiser-style demon, (but instead of pins on his head he has buzz saws). Then there's the ballerina demon with the mouth full of teeth for a face (that the credits call "the Sugar Plum Fairy"). The camera pulls back and there are thousands of these elevator cubes, each with a different kind of monster in them. This will be fun on dvd because you can't possibly see all the different kinds of monsters on one viewing of this movie...but I noticed a giant praying mantis, swamp creatures, giant bats, skeletons, lots of different kinds of aliens, killer robots, etc.
Dana realizes that she summoned the zombies but puts it together that there could have been other kinds of monsters to kill them , but she chose "their fate" -- similar to the Ghostbusters "choosing" the Stay Puft Marshmallow Man to destroy NYC.
The scientists order the two teens killed, but make sure to have the agents kill Marty first and then Dana. The facility's SWAT team is ordered to mobilize to head down to kill them....but a security guard opens the elevator and is startled by a zombie arm that was in there with the two teens...and Dana and Marty kill the guard and escape into the facility. They find a control room and lock themselves inside. A woman (who we'll soon see is played by Sigourney Weaver) comes on a PA system telling the kids that it's time for them to die and that there's a bigger picture they don't understand...and that she's sorry they have to die, but they need to be sacrificed. Dana doesn't like this and decides to play with the buttons in the control room and she ends up releasing all the monsters at once. The SWAT team is by the elevator banks as wave after wave of the monsters are released. There are giant bats, spiders, a giant snake, killer robots, alien creatures, a killer clown, a group of creepy people wearing masks like "The Strangers," lots of Romero style zombies, the Sugar Plum Fairy, werewolves, mummies, you name it.
After one load of these things kills a bunch of people, the elevators open again, and there's a new round, and another, etc. The monsters start attacking each other too, and it's a free for all.
Finally, a bat slams through the window of the control room and Marty and Dana have to flee. They start running down the corridors as the monsters are eating all the scientists. The three main ones we met at the beginning (Hadley, Sitterson, and Lin) are all in their control room where the power's failing and they realize that since the ritual has failed the world is going to end. Hadley is knocked to the floor in an earthquake that also destroys the doors, allowing monsters inside. A Merman slowly sneaks up on him and kills him....it's a nasty looking thing with a giant fish head, arms, and a mermaid-like tail and when it eats Hadley it spurts blood out its blowhole like a whale. That was funny that he wanted to see the Mermen attack and one ends up killing him. Lin meanwhile gets eaten by a giant thing with tentacles and Sitterson escapes underground via some kind of emergency escape tunnel.
As soon as he gets down there, he's stabbed by Dana who thought he was a monster. As he's dying, the last thing that Sitterson says is to Dana where he tells her to kill Marty to save the world.
Marty and Dana run...and end up in a giant stone room that's ancient and appears to be where the sacrificial carvings are located. Beneath them is the chamber where the ancient gods are sleeping. Sigourney Weaver appears and tells them that Marty and then Dana have to die in order to keep the peace and ensure the world's survival. Dana can live, but Marty has to die or else everyone alive will die a horrible death when the gods wake up. Dana decides to pull the gun on Marty and is about to shoot him when Marty sees a werewolf sneaking up on Dana from behind. The werewolf bites her, and Marty grabs the gun and shoots at the werewolf, chasing it away. Now it's a fight between Marty and Sigourney...and the zombie girl with the journal appears and has a hatchet...Dana lets her go after Marty, but before the zombie can kill him, Marty pushes Sigourney around so that it's Sigourney that dies instead. Marty then pushes both Sigourney and the zombie into the pit where the ancients are sleeping.
The chamber starts to shake and collapse.
Dana and Marty then sit there as the world starts to end. Dana apologizes for almost killing him and Marty apologizes for letting the werewolf get her. The chamber then collapses and a giant hand and arm emerge from the ground under the cabin, smashing it, to let us know that the ancient evil gods have awakened and will bring Hell on Earth destroying everything and killing everyone.
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