The film begins with a vibrant festivity celebrating the Techno Trolls, but their revelry is abruptly interrupted by the Hard Rock Trolls, led by the fierce Queen Barb. In a dramatic turn, Barb shatters King Trollex's DJ station with the overpowering magic of her guitar, seizing the Techno music string. As Poppy, still adapting to her new role as queen, receives an invitation from Barb for a party allegedly aimed at unifying their musical tribes, her father warns her about the hidden motives behind Barb's intentions. Despite his concerns, Poppy chooses to send a bright and optimistic response to Barb via Debbie, her pet bat.
Filled with enthusiasm, Barb reveals her sinister plan to conquer all forms of music by crafting an enchanted guitar featuring all six strings, intending to eradicate any musical competition and solidify rock as the sole genre. Determined to show her worth as a queen, Poppy sets out on an adventure in an air balloon to unite the various factions of Trolls, with Branch enthusiastically joining her to express his feelings. Unintentionally, Biggie becomes a stowaway on their journey.
Their first stop lands them in the realm of the Classical Trolls, only to find it devastated by Barb's forces and its citizens captured. A lone flute named Pennywhistle discloses the grim reality to Poppy, igniting her resolve to gather the Trolls against Barb’s oppression. Although Branch and Biggie plead for Poppy to return to safeguard the Pop string, Poppy reveals her clever concealment of it in her hair. Their travels lead them to the land of the Country Trolls, where their lively introduction through pop music lands them in jail under the watch of the disgruntled Delta Dawn. However, a Country Troll named Hickory, who claims to share Poppy’s sentiments, plots their escape, which leads them on a chase into danger.
As they traverse through peril, they encounter the bounty hunter Chaz, a Smooth Jazz Troll who has been misled into trying to capture Poppy on Barb's orders. After a tussle with Chaz, Biggie, feeling disillusioned with Poppy's leadership, decides to abandon the quest. Shortly afterward, the group is taken aboard a spaceship filled with magical soap bubbles, where they find the Funk Trolls. Here, Poppy discovers that Cooper, previously believed to be an outsider, is actually the lost twin prince of the Funk tribe. His parents, King Quincy and Queen Essence, share the history of discord: the original Pop Trolls had attempted to seize control of the music strings, leading to divisions that separated the tribes.
When the Rock Trolls hijack the Funk ship, the Trolls find themselves in peril, forcing Cooper to eject them in bubbles while unintentionally causing a rift between Poppy and Branch. Their argument culminates in Poppy's unwillingness to return home, leading to heartbreak as they part ways, both feeling lost. Branch, captured by bounty hunters from K-Pop and Reggaeton, cleverly persuades them to abandon their efforts and help rescue Poppy instead.
Reunited with Hickory, Poppy reveals the hidden Pop string. However, the twist unfolds that Hickory is a Yodel Troll, working with Dickory to initially capture her for Barb. Despite this betrayal, Hickory has a change of heart. Poppy soon finds herself in direct confrontation with Barb, who holds the sixth string captive.
Biggie, witnessing the devastation of their home upon his return, rallies their friends to rescue Poppy and Branch, leading them into the Rock Troll territory where Barb has captured Poppy and other Troll leaders to demonstrate her dominance using her enchanted guitar. In a critical moment during a concert, Poppy, seeming to succumb to Barb's spell, surprises everyone by pulling a fast one, crafted by her gumdrop earplugs, and shatters the guitar. The destruction of the guitar releases the captured Trolls but also destroys the musical strings, leaving the Trolls devoid of color — a blow to Barb’s ambition.
As silence follows, Cooper hears his heartbeat, transforming it into a rhythm that quickly gathers participation from all Trolls, who help restore their music through improvised sounds. Guided by Poppy's encouragement, all Trolls, including the sorrowful Barb, start singing together, rediscovering their colors and embracing their unique musical identities. In a touching conclusion, Branch finally confesses his love to Poppy, who reciprocates, culminating in a majestic performance celebrating their diversity and unity.
As the credits reveal a whimsical mid-scene, Bridget and King Gristle arrive in the Pop Troll village, blissfully unaware of the extraordinary events that unfolded, anxiously hoping they haven't missed the grand celebration.
