Residents of the small, isolated, 19th-century Pennsylvania village of Covington live in fear of "Those We Don't Speak Of," nameless humanoid creatures living within the surrounding woods. The villagers have constructed a large barrier of oil lanterns and watchtowers that are constantly staffed. After the funeral of a child, the village Elders deny Lucius Hunt's request for permission to pass through the woods to get medical supplies from the towns. Later, his mother Alice scolds him for wanting to visit the towns, which the villagers describe as wicked. The Elders also appear to have secrets, keeping physical mementos hidden in black boxes, supposedly reminders of the evil and tragedy in the towns they left behind.
After Lucius makes an unsanctioned venture into the woods, the creatures leave warnings in the form of splashes of red paint on all the villagers' doors.
Ivy Elizabeth Walker, the blind daughter of Chief Elder Edward Walker, informs Lucius that she has strong feelings for him and he returns her affections. They arrange to be married, but Noah Percy, a young man with an apparent developmental disability, stabs Lucius with a knife, because he is in love with Ivy himself. Noah is locked in a room while a decision awaits regarding his fate.
Edward goes against the wishes of the other Elders, agreeing to let Ivy pass through the forest and seek medicine for Lucius. Before she leaves, Edward explains that the creatures inhabiting the woods are actually members of their own community wearing costumes and have continued the legend of monsters in an effort to frighten and deter others from attempting to leave. Ivy and two young men are sent into the forest, but they abandon Ivy almost immediately, fearful of the creatures, rationalizing they will spare Ivy out of pity. While traveling through the forest, one of the creatures suddenly attacks Ivy. She tricks it into falling into a deep hole to its death. The creature is actually Noah wearing one of the costumes, which he discovered under the floorboards of the room where he had been confined after stabbing Lucius.
Ivy eventually finds her way to the far edge of the woods, where she encounters a high, ivy-covered wall. After she climbs over the wall, a young park ranger named Kevin spots Ivy and is shocked to hear that she has come out of the woods. The woods are actually the Walker Wildlife Preserve, named for Ivy's family, and that it is the early 21st century instead of the 19th century as the villagers believe. Ivy gives Kevin a list of medicines that she must acquire, also giving him a golden pocket watch as payment.
During this time, it is revealed that the village was founded in the late 1970s. Edward Walker, then a professor of American history at the University of Pennsylvania, approached other people he met at a grief counseling clinic, all suffering the crime-related death of loved ones. He asked them to join in creating a place where they would sustain themselves and be protected from any aspect of the outside world. They built Covington in the middle of a wildlife preserve purchased with Edward's family fortune. The head park ranger tells Kevin that the Walker estate pays the government to keep the entire preserve a no-fly zone, while also funding the ranger corps who ensure no outside force disrupts the preserve. Kevin discreetly retrieves the requested medicine from his ranger station and Ivy returns to the village with the supplies, left unaware of the truth of the situation.
During her absence, the Elders secretly open their black boxes, each containing mementos from their lives in the outside world, including items related to their past traumas. The Elders gather around Lucius's bed when they hear that Ivy has returned and that she killed one of the monsters. Edward points out to Noah's grieving mother that his death will allow them to continue deceiving the rest of the villagers that there are creatures in the woods. Ivy comes in and tells Lucius that she has returned.
Bryce Dallas Howard as Ivy Elizabeth Walker
Joaquin Phoenix as Lucius Hunt
Adrien Brody as Noah Percy
William Hurt as Edward Walker
Sigourney Weaver as Alice Hunt
Brendan Gleeson as August Nicholson
Cherry Jones as Mrs. Clack
Celia Weston as Vivian Percy
Frank Collison as Victor
Jayne Atkinson as Tabitha Walker
Judy Greer as Kitty Walker
Fran Kranz as Christop Crane
Liz Stauber as Beatrice
Michael Pitt as Finton Coin
Jesse Eisenberg as Jamison
M. Night Shyamalan as Guard at Desk
Charlie Hofheimer as Kevin Lupinski
Joey Anaya and Kevin Foster as Those We Don't Speak Of