Poppy, the relentlessly upbeat and optimistic queen of the Pop Trolls, discovers that her colorful world is but one of six different Troll tribes scattered across the land, each dedicated to a different genre of music: Funk, Country, Techno, Classical, Pop, and Hard Rock. When a mysterious invitation arrives suggesting that the Troll kingdoms are united in harmony, Poppy's curiosity is piqued. Meanwhile, Branch, her best friend who is just beginning to confess his feelings for her, is skeptical of the outside world and wary of the true intentions behind the invitation.

Unbeknownst to Poppy, the queen of the Hard Rock Trolls, Barb, plans to steal six magical strings from each Troll tribe that represent their specific music genres. Her goal is to destroy all other kinds of music and ensure rock reigns supreme. Resolute in her belief that bringing all Trolls together in harmony is possible, Poppy embarks on a mission accompanied by Branch and her friends to visit the other lands, hoping to unify the disparate tribes. They set out on an adventure that unveils the rich diversity and cultural differences among the Trolls, while also revealing the looming threat posed by Barb's ambition. As Poppy presses forward with her journey, she learns the importance of understanding and respecting all forms of music and the dangers of imposing uniformity.