Tobias Tidemann tells his daughter Nora Tidemann the story of a time when humans and trolls lived in harmony, until the Christian King Olaf forced the trolls into sunlight, wiping out most of Norway’s trolls and imprisoning the Troll King and his offspring underground.
Thirty years later, Nora lives alone in a remote cabin, three years after her encounter with the Troll King, having stepped away from her job studying trolls for the Norwegian government. She is visited by Andreas Isaksen, who asks for her help with a new discovery. They travel to Vemork and enter a top-secret underground research facility run by Marion, which houses a hibernating troll codenamed “Jotun,” as well as ancient documents written by Olaf. While reuniting with Professor Møller and Professor Wangel, Nora approaches the sleeping Jotun while singing a song taught to her by Tobias. Jotun awakens and breaks free, killing Wangel and escaping from the facility, as Møller escapes with a sample from Jotun.
Nora, Andreas and Marion meet Major Kristoffer Holm and Norway’s Prime Minister Brichmann, who orders that the troll be killed despite Nora’s objections. The trio convince Kris to take them along on his operation, using two helicopters equipped with powerful ultraviolet lights. Jotun attacks a ski resort, killing numerous civilians, before Kris’s forces arrive. The UV lights prove effective, but Jotun manages to destroy one of the helicopters, killing Kris’s friend Amir, before departing.
At Nora’s recommendation, the team return to the Dovre Municipality mountain cave where the Troll King emerged and find the underground pit that was once used to trap him. Nora lures another troll out of hiding: the now-adult offspring of the Troll King. She pacifies the young troll using the same song that awakened Jotun and nicknames him “Beautiful.” She suggests that they attempt to have Beautiful communicate with Jotun, hoping to prevent any further conflict.
Nora and the team manage to bring Jotun and Beautiful together, and the two trolls communicate, but Jotun attacks Beautiful and causes him to fall into a deep fjord, forcing the team to flee. As the team regroup, a regretful Nora reveals that she had already found Beautiful and communicated with him once before. Marion deduces that the vengeful Jotun is following an ancient pilgrimage route which will lead him to Trondheim, Olaf’s former capital.
In Trondheim, the team discover Olaf’s hidden tomb in Nidaros Cathedral. Inside, they find a missing piece of an ancient document, which reveals that Olaf had a change of heart and decided to give the trolls a sanctuary where they could live in peace, but was killed on the Church’s orders so they could continue to wipe out the creatures. Nora also realizes that holy water can be used against the trolls, and the team recruits some of Trondheim’s citizens to help prepare a trap for Jotun.
Jotun enters Trondheim and approaches the cathedral, sustaining some damage from the team’s new holy-water weapons but ultimately breaking through their defenses. However, Beautiful also arrives and challenges Jotun, igniting a battle between the two trolls until Jotun gains the upper hand. Nora and Marion distract Jotun with holy-water rockets, while Andreas sacrifices himself by diving into Jotun’s mouth with their last holy-water bomb, which detonates inside Jotun and badly wounds him. Beautiful then kills Jotun by punching through his torso and ripping out his heart.
Later, Nora, Kris and Marion now live with Andreas’s wife Sigrid and infant daughter Uhura, in a new sanctuary that has been established for Beautiful. In a mid-credits scene, Møller is secretly studying a baby troll that has been grown from one of Jotun’s genetic samples.
