Picking up from the thrilling conclusion of season 2, titled The Stone Forest, Hilda finds herself in a precarious situation after being transformed into a troll. Desperate to escape her troll mother, Trylla, Hilda ventures out only to be petrified by the harsh sunlight as she leaves the Stone Forest. Meanwhile, back in Trolberg, Johanna stumbles upon Baba in Hilda's absence and, with the help of Tontu, deduces that Hilda has fallen victim to a changeling spell. Realizing the gravity of the situation, Johanna teams up with Baba, Tontu, Alfur, and Twig to search for Hilda, but their efforts to find the entrance to the Stone Forest prove futile.
At the same time, Erik Ahlberg holds a press conference announcing a curfew due to rising troll activities near Trolberg. As his report airs, David's mother becomes intrigued by an ad inviting people to join the Trolberg Safety Patrol. Later on, students at Edmund Ahlberg Elementary School are shown a troubling safety video that misrepresents trolls, which leads to protests from Frida and David.
As night falls and Hilda remains unaccounted for, Johanna bumps into a group of Safety Patrol officers and reluctantly heads home. Once darkness envelops everything, Hilda manages to unpetrify but finds herself unable to return because of the ominous belltowers. While trying to escape, she encounters a massive cave adorned with bells, where she meets a giant troll named Trundle. Trundle promises to help Hilda regain her human form in exchange for completing a series of tasks. He reassures her that Trylla holds no ill will, prompting her to return to her.
Together, they hide in the Stone Forest to avoid the sunlight. During their escape, Hilda becomes aware of an alarming sound that only trolls can hear, a distress signal that has been growing stronger, calling them towards Trolberg.
Meanwhile, Frida and David uncover Hilda's plight when Baba manages to escape from Tontu's supervision. Frida, despite [Kaisa]’s (the talented librarian skilled in witchcraft) warnings, attempts a body swap spell that ultimately fails. As night returns, Hilda embarks on her first task from Trundle, which involves secretly delivering a cup of troll mead. Trundle further instructs her to remove the bells that bind him to the cave.
Later, Trylla reveals to Hilda the remnants of the Mountain King's castle—a giant troll who had sought to lead troll attacks against Trolberg but was defeated. As the changeling spell cannot be reversed, Hilda resolves to aid Trundle. She uses pillows from his treasure stash to silence the bells. Meanwhile, Johanna and Alfur make a breakthrough by discovering the entrance to the Stone Forest, only to be thwarted when trolls block their path, forcing them into a frantic escape.
With dawn approaching, Johanna reluctantly seeks Erik Ahlberg's assistance as a final option. Frustrated and yearning to help Hilda, Frida proposes that she and David sneak outside the city to magically connect with a troll, reminiscent of her previous encounter with the Kraken's mother in season 2. In the meantime, Trundle assigns Hilda her final task: retrieving a large red orb hidden in the ruins of the Mountain King's castle. When Hilda touches the orb, she is transported into a surreal vision involving a colossal creature resembling her mother, looming over Trolberg. Upon returning to reality, she attempts to deliver the red orb to Trundle, but is suddenly attacked by a massive bearded troll.
Outside, Ahlberg, Johanna, Baba, Alfur, and Tontu reach the entrance to the Stone Forest. Just as Baba opens it, Hilda emerges with the bearded troll hot on her heels. A chaotic battle ensues, resulting in Ahlberg losing consciousness and Hilda inadvertently causing the troll to smash Johanna's car before plummeting into a ravine. Ultimately, Hilda and Johanna reunite and present the orb to Trundle's lair. Remarkably, Trylla appears and is reconnected with Baba, reversing the changeling spell and restoring Hilda's humanity, while Baba is transformed back into a troll. In a shocking turn of events, Trundle breaks free and reveals that the orb is his eye.
Now that he can see, it becomes clear that he is the Mountain King, rallying the trolls to launch an assault on Trolberg. In the city, David distracts his mother in the belltower, allowing Frida to initiate a mental communication with one of the trolls. She experiences a haunting vision, mirroring Hilda’s, of the giant troll-like being resembling her mom attacking the city. As Trundle wreaks havoc by destroying the belltowers, he is confronted by his brother, the bearded troll, who he refuses to fight. Trundle's followers restrain his brother to allow for broader destruction.
The Safety Patrol, led by Ahlberg, emerges to fend off the impending attack. Hilda, recalling the Mountain King's warning about the sleeping mother troll beneath the city, realizes that if the Safety Patrol acts aggressively, the mother will awaken and retaliate, risking Trolberg's destruction. Hilda begs Ahlberg to reconsider, but he remains obstinate and plans to fire upon Trundle with the Safety Patrol's cannon. In a desperate bid to intervene, Hilda hurls Trundle's eye into Ahlberg's face, prompting him to witness the devastating vision that haunts Hilda. Now understanding the potential catastrophe ahead, Ahlberg commands the Safety Patrol to stand down.
As the mother troll settles back, the trolls move toward the heart of the city. With Frida's magical assistance, they unveil her identity: Amma, the ancient matriarch of all trolls. Having slept for centuries to avoid causing disruptions for the humans above, Amma persisted in her dormant state. Though the trolls used to maintain a connection with her, their continued separation led to misunderstandings and conflict. Now close enough, the trolls can finally hear Amma's voice, bringing forth new life as they sprout vegetation across their bodies.
In the aftermath, Ahlberg resigns after receiving a medal for managing the crisis, passing control of the Safety Patrol to his deputy, Gerda. Gerda promptly declares a "Night of the Trolls", where trolls will be permitted into the city annually to reconnect with Amma. While some humans may still harbor fears toward trolls, this yearly gathering fosters healing and gradually reduces the mutual trepidation between the two species. Hilda and Baba's families remain connected, as Baba visits Hilda’s home from time to time, and Hilda occasionally spends time in the mountains with Trylla and Baba.
