Plot
Picking up from the season 2 finale "The Stone Forest", Hilda, now a troll, runs away from Trylla, her troll mom, and is petrified by the sunlight when she leaves the Stone Forest. Meanwhile, in Trolberg, Johanna discovers Baba in Hilda's place, and Tontu deduces Hilda had been the victim of a changeling spell. Realizing what this means, Johanna, Baba, Tontu, Alfur, and Twig go searching for Hilda but are unable to locate the entrance into the Stone Forest.
Elsewhere, Erik Ahlberg holds a press conference in which he announces a curfew because of the increasing troll activity outside Trolberg. While watching, David's mom becomes intrigued by an advertisement to volunteer for the Trolberg Safety Patrol. Later, the students of Edmund Ahlberg Elementary School are shown a safety video about trolls, but Frida and David protest the inaccurate information portrayed in the video.
As evening approaches and with still no sign of Hilda, Johanna runs into a group of Safety Patrol officers, and reluctantly returns home. After sunset, Hilda unpetrifies, and finds herself unable to return to Trolberg because of the belltowers. As she retreats, Hilda stumbles upon a huge cave with dozens of bells hanging from the top of the entrance. Inside, she finds a gigantic troll, who calls himself Trundle. He tells Hilda he will help her become human again if she completes some tasks for him. He also assures her that Trylla means no harm, and Hilda reluctantly returns to Trylla. They retreat into the Stone Forest to escape the rising sun. When the two arrive outside the mountains, Hilda "feels" a strange sound, which Trylla explains is something that all trolls hear, calling them to the city of Trolberg, and that the call has been growing stronger lately.
Frida and David discover Hilda's predicament when Baba escapes from Tontu's care and so Frida, against the advice of Kaisa, the librarian skilled in witchcraft, unsuccessfully attempts a body swap spell. Night falls again, and Hilda begins the first of the tasks given to her by Trundle by secretly bringing a cup of troll mead to him. Trundle then instructs her to remove the bells that trap him in the cave.
Later, Trylla shows Hilda the ruins of the castle of the Mountain King, a giant troll who tried to rally the trolls into attacking Trolberg, but was defeated. Hilda doubles down on helping Trundle after Trylla tells her that the changeling spell cannot be undone. She uses pillows, stolen from a troll's treasure collection, to silence and remove the bells. Elsewhere, Johanna and Alfur find the entrance to the Stone Forest, but the trolls close it before they can enter. The two are then attacked by a group of aggressive trolls and narrowly escape.
The following morning, Johanna begrudgingly seeks Erik Ahlberg's help as a last resort. Frustrated by being unable to help Hilda, Frida suggests that she and David sneak out of the city so she can try magically communicating with a troll like she did with the mother Kraken in season 2. Meanwhile, Trundle sends Hilda on her final errand: to bring him a large red orb, hidden in the ruins of the castle among the hoard of the Mountain King. When Hilda touches it, she is thrown into a strange vision in which a giant creature resembling her mom looms over the walls of Trolberg. Hilda is then whisked back into reality, and she attempts to bring the red orb back to Trundle, only to be attacked by a giant bearded troll.
Outside, Ahlberg, Johanna, Baba, Alfur, and Tontu arrive at the entrance to the Stone Forest. Just as Baba opens the entrance, Hilda rushes out, chased by the bearded troll. A fight ensues, ending with Ahlberg being knocked unconscious and Hilda causing the troll to accidentally crush Johanna's car and fall into a ravine. Hilda and her mom are reunited and they bring the orb to Trundle's cave. Trylla, having followed them, appears and is reunited with Baba, reversing the changeling spell, and making Hilda human again, and Baba a troll again. Suddenly, Trundle bursts from his prison, unveiling the orb to be his eye. With his sight now restored, it is revealed that he is the Mountain King, and he incites the trolls to storm Trolberg.
In Trolberg, David distracts his mom in the belltower so Frida can make a mental connection with one of the trolls, and she sees a similar vision to the one Hilda saw; a giant troll-like being resembling her mom, attacking the city. Meanwhile, Trundle begins destroying the bell towers but is halted by the arrival of the bearded troll, revealed to be his brother. Refusing to fight him, Trundle has his followers restrain his brother before breaching the city wall.
The Safety Patrol arrives to fight off the attack, with Ahlberg taking back control. Hilda, remembering the vision from Trundle's eye, realizes that the mother of all trolls lies underneath the city, and she will rise up to defend her children if the Safety Patrol attacks them, destroying all of Trolberg in the process. Hilda attempts to prevent the attack, but Ahlberg refuses to listen to her, and uses the Safety Patrol's cannon to destroy Trundle, awakening the mother troll. In a last-ditch attempt to stop him, Hilda takes Trundle's eye and throws it into Ahlberg's face, causing him to see a similar vision to what Hilda saw. Now understanding what is at stake, Ahlberg orders the Safety Patrol to stand down.
The mother troll stops trying to get up, and the trolls all walk peacefully to the center of the city where she rests. Using Frida's magic, Hilda and Frida learn her identity: Amma, the mother and grandmother of all trolls, having laid dormant for centuries so as to not disturb the humans who built Trolberg above her. For a while, the trolls could still reconnect with their buried mother, but then the settlers, unaware of Amma's presence and thus why the creatures kept wandering into the city, drove them away, eventually erecting a wall to keep them out. Now that the trolls are close enough to Amma, they can finally hear her voice again, causing them to sprout plants all over their bodies.
In the aftermath, Ahlberg resigns after accepting a medal for his eventual handling of the crisis and relinquishes control of the Safety Patrol to his deputy Gerda Gustav. Gustav immediately announces a "Night of the Trolls", in which the trolls are allowed into the city once a year to reunite with Amma. While some humans still fear the trolls and vice versa, the annual peaceful exposure is gradually eroding the mutual fear between the two species. Hilda's and Baba's families still maintain contact with each other, with Baba occasionally visiting Hilda's home and Hilda occasionally spending a night in the mountains with Trylla and Baba.
