On Halloween night, two young girls come to the house of their neighbor, Miss Turner. She sneaks up behind them and jokingly surprises them, and then invites them inside for candy. A coven of witches have gathered in the living room and Miss Turner uses a spell to keep the girls from running. Miss Turner begins chanting as well and the coven's leader, Robbie Collins, uses her own blood as ink to write in a book. She tells Miss Turner that her boyfriend Brian chose another woman and now she's going to make sure he and his friends rot in hell. Robbie daubs blood on the foreheads of the two girls and gives them the book, which has a circle on the cover. The witch tells them that it won't hurt but hundreds of black tendrils rise out of the floor and grab the screaming girls.
Brian and his wife Lisa have gone to their cabin in the Maine woods for a weekend alone. As they start to kiss, someone knocks at the door and Brian reluctantly answers it. It's his agent, Anita, who asks if his editor George Clayton is there. Lisa admits that George called ahead and said that he's coming and bringing Kate, Brian's publisher. As Anita goes to the bathroom to freshen up, Lisa admits that she thought she'd get Brian's editor, agent, and publisher there to help him overcome his writer's block.
Everyone gathers in the living room and Anita points out that Brian's first novel, Blood Thirsty under the pen name Robert Collins, was a best seller. However, his second book is overdue and Anita insists that they need it. Lisa tries to defend her husband but Anita says that it's time he has to go back to work. Brian takes offense, accusing them all of being there just for the money they'll make from him, but Anita reminds him that she's been there for him from the beginning. The writer apologizes and admits that he's had trouble sleeping.
As Kate starts to ask him how his writer's block started, there's a knock at the door at exactly 9:45. Lisa assures Brian that she didn't invite anyone else and they're too far out for trick-or-treaters. Everyone hesitates, filled with a sense of foreboding, but Anita finally goes over and answers the door. The two trick-or-treating girls from the beginning are at the door and hand her the book. When she looks up again, they've vanished. She takes it inside to show the others and they realize that it's a novel, titled The Circle. It has Brian's pen name on the cover and the others figure that Brian has already finished his second book and had the girls deliver it as a stunt. Brian insists that he has no idea what it was, and Anita starts reading it. The story starts with a group of friends gathering at the cabin of a writer on Halloween to help him get over his writer's block, and then says that no one was prepared for what would happen next.
Brian accuses the others of playing a prank on him and tells Anita to stop reading, but she continues while the others deny knowing about it. In the story, the circle of gathered friends didn't recognize the book's power at first, but they soon would when a cloud of darkness encircled the cabin, trapping them inside forever. George goes to the door and discovers that the events in the book are coming true. A cloud of inky blackness has surrounded the cabin, and George goes outside to investigate. The others tell him to come in, but George thinks it's some kind of a joke... until a tendril of the black substance grabs him and pulls him into the surrounding darkness, splatting black goo on Kate. The others pull her inside and slam the door shut just as another tendril slams into it.
Inside, Anita tries to calm down Kate and Brian checks to make sure that none of the darkness got into her. The cabin has no cell phone reception and they can't call for help, and the cabin starts to shake as the darkness slams into it. Brian realizes that everything in the book is coming true and Lisa reads from it. Next it describes how the writer's story comes to life and the darkness killed the editor. Brian tells the others that the darkness creature is from his first story, Blood Thirsty, and it's coming to life just as the novel says.
The others see George at the window, but he's covered in the darkness which has twisted his body. He claws at the windows, spewing the black goo out of his mouth, and Brian orders everyone back. The writer warns them that in his story, the darkness possesses its victims and turns them into bloodthirsty creatures that feed on the living. Kate suggests using light as a weapon against it, while Anita insists that it can't be happening and a sobbing Kate tells her that it is.
The darkness starts oozing in beneath the doors and windows and Brian, Lisa, and Anita try to block it as best they can. Kate collapses in a corner, sobbing, as they realize that nothing short of an airtight seal will keep out the darkness. He tells them to put lights in front of all the gaps but Kate says that it doesn't matter because their fates have been decided. Anita picks up the book and reads, and discovers that Kate is saying the same words as in the book. Kate grabs the book and throws it into the fire, but they watch in horror as they realize that it won't burn. When Brian tries to pull it out with a poker, it disappears and reappears, unharmed, on the kitchen table. He stares at it and mutters that "she" cursed it.
Lisa picks up the book and reads to the end to find out what happens. The story concludes with everything going back to the way it began at 9:45. Kate starts sobbing again and then vomits up the liquid darkness. Brian warns that even a drop of the darkness getting into someone can transform them into one of the Blood Thirsty, and Kate got a drop in her mouth. Kate turns into a zombie and tries to attack Anita, but Brian distracts it while Lisa grabs a flashlight and drives Kate back. They force her into the bathroom and lock the door, Brian is splattered with the darkness from Kate but checks and makes sure that none of it got into him.
As they make sure the bathroom door is secure, Brian warns them that there's no hope for the infected as the darkness devours them from within. They try to work out a way to escape but Brian warns that the car is too far away. Anita notices a drop of the darkness on her head and absently wipes it off. Meanwhile, Brian assures them that the infected people can be killed but Lisa is reluctant to do so. Anita is initially willing but realizes she can't bring herself to do it. Brian tells them that in his novel, the creatures are also destroyed by sunlight, but warns that they'll never survive until sunrise.
With time running out, Brian suggests that they rewrite the story and change the ending. Anita doesn't see how it can work but Brian points out that they don't have any other options. They hear Kate break down the door of the bathroom and lock themselves in the bedroom, locking the door. Kate pounds on the door for a while and then goes away, and Brian says that they have to get to the book in the living room. The electrical lights go out and Brian tells Lisa and Anita to start a fire from the furniture and hold out until sunrise. He grabs a cane and goes out, and Kate jumps him from behind.
The two women hear Brian scream and Lisa turns to Anita... only to discover that she has transformed into a Blood Thirsty. Anita lunges at Lisa, but Brian arrives and beats her to death. He's managed to kill Kate as well, but has become infected. As the darkness crawls on and inside of his body, Brian takes Lisa to the living room and tells his wife to tie him up before the darkness takes him over. Once he's secured, Lisa brings him the book but Brian is unable to write because of his block. Lisa points out that the book said that Brian would find his muse, and he concentrates and starts writing. However, his pen doesn't work and none of their other pens will make a mark on the book. Brian realizes that "she" used blood to write in the book and Lisa asks who "she" is.
Before Brian can explain, someone knocks at the door. Lisa goes to answer it and Robbie bursts in, holding a knife to Lisa's throat. Brian begs her to let his wife go, but Robbie ignores him and asks what he thought of her story. She explains that she used her witchcraft to help Brian write his first book, but he never fulfilled his end of their bargain so she has vowed to destroy his world. Robbie throws Lisa to the ground and says that Brian loves her and will come back to her. She made him famous but he refused to leave Lisa like he promised.
As Robbie gloats, Lisa grabs the poker from the fireplace and knocks her out. She goes to her husband, who says that he was trying to make their life better and get away from Robbie. Disgusted, Lisa tells him to finish the book and then defends herself as Robbie wakes up and attacks. As the darkness continues slamming into the cabin, Brian jabs the blood into his own wounds and starts writing. Lisa manages to grab Robbie's knife and kill the witch with it, and then goes over to Brian. The darkness has taken him over and she picks up the book to see how he's rewritten it. The last few words are the same thing that it said earlier, that everything ended and then returned to what it was at 9:45 on Halloween. Lisa pages through the book and finds the same words written over and over again, hundreds of times. As the cabin walls start to collapse, the Blood Thirsty Brian rips free and attacks Lisa...
At 9:45, there's a knock at the door and Anita finally goes to answer it. The two girls give her the book and disappear. The darkness comes, killing everyone... and things reset to 9:45 and happen again... and again... and again, for all eternity.