Tina, Eva Melander works for the Swedish Customs Service and uses her heightened olfactory sense to detect contraband as well as human emotions like guilt and shame. She carries a strong Neanderthalic look and lives in a secluded house in the woods with Roland, Jörgen Thorsson the dog trainer. At the border one day, she detects a memory card filled with child pornography, and her superior asks her to assist with the ensuing investigation.
The following day, a strange man bearing similar facial features to Tina appears at customs. His bag contains maggots and a device he calls a maggot incubator. Tina lets him pass, but he soon returns and offers to be strip searched. The man reveals female genitalia and a long scar on his tailbone and introduces himself as Vore, announcing that he will stay in a nearby hostel.
Tina visits a nursing home to see her father, who does not look like her. When asked about the scar’s origin, he tells her she fell on something as a child. She then goes to the hostel, where she finds Vore eating maggots off a tree. He gives her one, and she consumes it. She offers him a room in her guest house, and he accepts. Tina brings him home, and he makes an unsuccessful move to kiss her. Roland grows suspicious.
During a police search of an apartment inhabited by suspected pedophiles, Tina accompanies the officers and detects a camera with footage of an infant being sexually abused. The occupants are arrested, but the authorities cannot identify who is trafficking the infants.
In a thunderstorm, Vore enters Tina’s house and the two huddle under a table, terrified by the lightning, before sharing a kiss. Tina later reveals that she has a chromosome deformity that makes sex difficult and prevents her from bearing children. Vore tells her that it isn’t a deformity, and urges her to ignore human judgments. Tina is astonished when an erect penis emerges from her groin. The couple makes love, and Vore explains that Tina is a troll, like him, and that he hopes to reach a hidden group of trolls in Finland.
Emboldened by her new identity, Tina begins to live more like a troll and even evicts Roland. She discovers that Vore has taped his refrigerator shut; when she opens it, she finds an infant in a cardboard box. Vore explains that the baby is a hiisi, an unfertilized troll embryo that will soon die, and that he intends to use it as a changeling, secretly replacing a real human infant with the dying troll embryo.
When one of the arrested pedophiles is being moved, Vore intercepts the van and kills him to prevent him from telling the police that Vore trafficked infants. He confesses that the infant trafficking is part of a troll plot to take revenge on humans for torturing trolls in the 1970s. Tina is troubled, believing that past human mistreatment does not justify such vengeance.
The next day, neighbors call an ambulance for a sick baby—unbeknownst to them, it is a changeling. Realizing Vore’s deception, Tina heads to the guest house. Vore and his belongings are gone, but he leaves a note directing her to meet him on the ferry. She finds him on the deck and explains that compassion toward humans does not make her human, and that trolls can be capable of mercy too. She signals the police, who close in and arrest Vore, but he escapes by leaping overboard; his body is never found.
Tina’s father finally reveals the truth: he once worked as a caretaker at a psychiatric hospital where trolls were tortured and experimented on; he adopted Tina, whose original troll name was Reva, to raise her as a human. Her real troll parents are dead, and he knows where they are buried. Later, Tina drives to the old hospital and discovers her parents’ graves.
Months later, Tina finds a parcel on her porch containing a troll infant and a postcard from Finland.
