Plot summary

In a nightmare, Sophie relives her mother reading to her from a supernatural storybook called 'The Book of Monsters' when Sophie was eight years old. A shapeshifting creature manifests under the bed and kills Sophie's mum.Sophie's father Jonas gifts Sophie with the ancient book for her eighteenth birthday. Jonas then leaves his daughter alone in the house to host a party with her friends Mona and Beth.Sophie's rival Arya disrupts the party by inviting numerous people Sophie doesn't know. Other party guests include classmate Gary, German exchange student Helga, virgin Dave, Arya's bully boyfriend Brice, and Sophie's crush Jess.Pandora, a mysterious woman in a red dress, seduces Dave into coming with her to Sophie's bedroom. Sophie has a private conversation with Jess to question why she is friends with Arya.Arya humiliates Sophie by having a stripper named Carl give her a lap dance in front of everyone. Arya further embarrasses Sophie by revealing she stayed in a mental institution after claiming a monster murdered her mother. Sophie punches Arya.Pandora bites Dave's neck and draws a bloody pentagram on his chest while morphing into the shapeshifter that killed Sophie's mum. Dave's virgin blood splashes onto the Book of Monsters.A supernatural wind suddenly blows through the house. A creature called Grota appears and begins violently tearing apart party goers including Brice. Sophie hides with Beth. Arya calls the police after barricading herself in the kitchen with Carl. Gary flees outside with two party goers. Mona and Jess escape to Sophie's bedroom.Impersonating Dave, Pandora knocks Jess unconscious. Pandora does the same to Mona and then reforms in Mona's image.A monster wearing a plague doctor mask kills the two people with Gary. Gary sees the plague doctor unleash sentient slugs.Sophie tells Beth she knows Grota from her mother's book. Remembering that the creature has a weak spot on its back, Sophie stabs Grota with a cake knife and kills it.The slugs possess garden gnomes that infiltrate the kitchen. Gary returns to the house in time to rescue Carl from the gnomes. Gary, Arya, and Carl then regroup with Sophie and Beth.Impersonating Mona, Pandora helps Jess recover. Pandora brings the book to Sophie.Officers Graham and Keith arrive outside. The plague doctor slaughters Keith and blows up the patrol car with Graham in it.With notes