Nightborn

Saga and Jon visit a derelict house in the Finnish forest that belonged to Saga's late grandmother. They find a tree growing through the floorboards of the bedroom Saga shared with her sister, Taru. Jon proposes to renovate the house into a family home where they can raise children together. They go into the surrounding forest, where Saga embraces a tree with a trunk resembling a pregnant belly, and have sex.

Saga has a difficult childbirth, severely tearing her birth canal, during which the baby boy is unsettlingly silent. On the maternity ward, the infant cries only when the curtains are opened. Breastfeeding the baby leaves Saga's breast bloodied and swollen, and his body is covered with thick black hair.

At the now-renovated house, Saga's childhood bedroom has been transformed into a nursery. When Saga's mother and Taru visit, Saga expresses concern that something is wrong with the baby, referring to him only as "it". Her mother dismisses these fears, even after the infant forcefully tears an earring from her earlobe.

At a naming ceremony in the garden, guests are unsettled by the baby’s appearance and visceral crying. A gust of wind from the forest blows a cover over his stroller, shielding him from the sunlight. Saga announces that the baby will be named Kuura, upsetting Jon, who had expected to name him Christian after his father. Saga notices the sunlight has left deep burns in Kuura's skin.

Kuura begins to behave very unusually. One night, Saga falls asleep while trying to settle Kuura and awakens to find he has escaped his cot and is transfixed by the tree that has begun growing again through the nursery floorboards. The following morning, Jon discovers the floorboards torn up and Saga asleep among the tree's roots, cradling Kuura. When Saga accidentally cuts herself on broken glass, she finds Kuura sucking up her blood. At a baby clinic, a clinician dismisses Saga's concerns and tells her that Kuura is simply advanced for his age, referring her for counselling to help her bond with him. Saga begins secretly feeding Kuura blood instead of milk.

While helping Jon dismantle rusted cars in the garden, Taru recounts her grandmother's warning that the surrounding forest is dangerous and explains that she kept scrap metal around the property because "cold iron bursts them". When Jon's parents visit, his father - a vicar - blesses Kuura with holy water, which has no physical effect on him, but later, when Kuura grasps an unused, cold fire iron, his hand is badly burned.

Jon and Saga’s relationship becomes increasingly strained when he expresses concern that Saga is failing to socialise Kuura properly. When he discovers she did not pursue the clinician’s referral, he storms out of the house, leaving Saga alone with Kuura. Saga awakes to find Kuura has bitten her. She locks herself in the bathroom while Kuura tries to get in, and tells her mother on the phone she feels she needs to kill Kuura before he kills her, but she tells Saga that there is nothing out of the ordinary.

When Jon returns to the house, he suggests Saga is exhausted and needs time to herself. She makes an overnight trip to the city to visit friends, leaving Jon to care for Kuura, but when separated from her son develops a severe rash which disappears only when she rushes home to be with him.

Kuura becomes increasingly violent. During a visit from Taru and her daughter Anna, he bites off Anna's finger. Jon tries to feed Kuura baby food but he shoves the metal spoon away and throws the jar at Jon. Saga gives Kuura raw meat instead, which he eats, and Jon accuses Saga of turning Kuura into a monster. Jon attempts to leave the house with Kuura, leading to a violent struggle with Saga. During the confrontation, tree branches erupt through the nursery floor and windows, fatally impaling Jon through the chest. Saga and Kuura feed on Jon's blood and heart.

In the aftermath, Kuura runs into the garden, where a humanoid figure separates itself from one of the trees and approaches him with outstretched arms as Saga watches from the porch. Kuura giggles and turns back to Saga, smiling. Saga smiles back.