Plot

When the Queen of the Hard Rock Trolls tries to take over all the Troll kingdoms, Queen Poppy and her friends try different ways to save all the Trolls.

Poppy and Branch discover that they are but one of six different Troll tribes scattered over six different lands devoted to six different kinds of music: Funk, Country, Techno, Classical, Pop and Rock. Their world is about to get a lot bigger and a whole lot louder. A member of hard-rock royalty, Queen Barb, aided by her father King Thrash, wants to destroy all other kinds of music to let rock reign supreme. With the fate of the world at stake, Poppy and Branch, along with their friends, set out to visit all the other lands to unify the Trolls in harmony against Barb, who's looking to upstage them all.

In the Techno Kingdom, King Trollex and his tribe of Techno Trolls are enjoying a vibrant rave when Queen Barb and her Hard Rock Trolls invade, wielding weaponized guitars. Barb demands Trollex surrender his "string," destroying the kingdom in her pursuit.

Meanwhile, in Pop Village, two years after making peace with the Bergens, Queen Poppy struggles to adjust to her role as leader of the Pop Trolls. She receives a letter from Barb, inviting her to bring her tribe's string to unite the trolls. Poppy's father, King Peppy, explains that there were once six magical lyre strings representing different music genres-Techno, Funk, Classical, Country, Hard Rock, and Pop. The trolls separated long ago after disagreements turned into conflict, each taking their string. Despite warnings from King Peppy, Branch, and others not to trust Barb, Poppy decides to prove her belief that all trolls are the same by sneaking off with the Pop string. Branch, who secretly wants to confess his feelings for Poppy, and Biggie, who stows away, join her on the journey.

Meanwhile, Cooper, inspired by illustrations of trolls resembling him, sets off on his own quest and is unexpectedly beamed into a spaceship. At the same time, Barb sends bounty hunters from smaller music genres to capture Poppy, promising to spare the tribe of the successful hunter.

Poppy's group arrives at the ruins of Symphonyville, home of the Classical Trolls, where they meet Pennywhistle, a sentient flute. She reveals Barb's plan to forcefully unite all trolls under Hard Rock. Undeterred, Poppy leads the group to warn the Country Trolls in Lonesome Flats. However, Poppy's attempt to "cheer up" the downbeat Country Trolls, led by Delta Dawn, backfires, and they end up imprisoned. They are rescued by Hickory, a smooth-talking Country Troll, who helps them escape.

After a canyon chase, Hickory builds a raft to take them to Vibe City, home of the Funk Trolls. Along the way, the group encounters Chaz, a Smooth Jazz Troll, who paralyzes them with his music. Hickory, using gumdrop earplugs, drives him away. Frustrated by Poppy's broken promises, Biggie leaves the group to return home.

Upon reaching Vibe City, revealed to be the spaceship where Cooper landed, the group reunites with him and discovers he is the long-lost son of King Quincy and Queen Essence, rulers of the Funk Trolls, and twin brother to Prince D. Poppy asks for their help against Barb, but the Funk royalty reveal the Pop Trolls' history of trying to unite all trolls under Pop, causing the original division. They emphasize that harmony requires respecting differences.

The Hard Rock Trolls attack, capturing the Funk string. Poppy's group is ejected to safety, but they are separated from Hickory. After arguing with Poppy over her refusal to listen, Branch storms off and is captured by Reggaeton and K-Pop Trolls. He convinces them to work together against Barb.

Hickory, revealed to be a disguised Yodeling Troll working with his brother Dickory, tries to steal Poppy's string but regrets his actions and warns her to flee. Barb arrives, captures Poppy, and takes the final string. Meanwhile, Biggie returns to find Pop Village in ruins and rallies the Pop Trolls to rescue Poppy, disguising themselves as Hard Rock Trolls.

At Volcano Rock City, Barb uses the six strings to turn the trolls into Rock Zombies during her concert. Branch, along with the Reggaeton and K-Pop Trolls, attempts a rescue but is zombified. Poppy, seemingly affected, reveals she used gumdrops to block the music. She confronts Barb for being a poor leader who ignores her people, smashing Barb's guitar and destroying the strings. Music ceases, turning all trolls gray.

Blaming Poppy for the loss of music, Barb laments history repeating itself. However, Cooper starts beat boxing, inspiring others to create rhythm. Poppy leads the trolls in singing from their hearts, restoring their colors and music. Barb accepts Poppy's friendship, and Branch confesses his feelings for Poppy. The trolls, united in celebrating their differences, return to Pop Village to party together.