[3] The film features the voices of Bella Ramsey, Ameerah Falzon-Ojo, Oliver Nelson, Daisy Haggard, Rasmus Hardiker, John Hopkins and Lucy Montgomery.

Production of Hilda and the Mountain King started in late 2019, and the film was produced by Silvergate Media and Mercury Filmworks.

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.

As evening approaches and with still no sign of Hilda, Johanna runs into a group of Safety Patrol officers, and reluctantly returns home.

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.

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, 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.

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.

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.

[8] The song When I Can't Sleep by Icelandic musical group Pascal Pinon is featured in the film, though it is absent from the soundtrack.

[17] On December 22, 2021, it was revealed that Giant Ant would be producing the opening credits of the film, along with new screen grabs and production assets.

[20] Aatqa Arham of Variety argued that while the storyline is "seemingly simple," it is filled with "emotional and philosophical wisdom," questioning the good and evil dichotomy, and said the film makes a statement that "the world...needs to change.

"[21] Joshua Fox of Screen Rant described the film as featuring "stunning visuals and music" and has a story which "elevates the source material to a tremendous degree.