Poppy discovers that Branch was once part of the boy band 'BroZone' with his brothers, Floyd, John Dory, Spruce and Clay. When Floyd is kidnapped, Branch and Poppy embark on a journey to reunite his two other brothers and rescue Floyd.
In a flashback, an infant Branch and his four older brothers-John Dory, Spruce, Clay, and Floyd-perform as a boy band called BroZone. Despite their talent, their clashing egos prevent them from achieving the "Perfect Family Harmony," a powerful ability Trolls can achieve when in perfect sync. Their inability to work together causes the band to break up, leaving Branch to be raised by his grandmother Rosiepuff.
In the present, Branch and Poppy are attending Bridget and Gristle's wedding in Bergen Town when they are interrupted by John Dory. John reveals that Branch's closest brother, Floyd, has been kidnapped by Velvet and Veneer, two ambitious Mount Rageon teenagers determined to become pop stars. Floyd has been trapped in a diamond perfume bottle that the siblings use to extract his essence, enhancing their voices but endangering Floyd's life. The only way to destroy the bottle is through the Perfect Family Harmony. Though hesitant, Branch agrees to help John Dory reunite their brothers and rescue Floyd. Along with Poppy and Tiny Diamond, they set off in John Dory's armadillo-shaped van, Rhonda.
Their first stop is Vacay Island, where they find Spruce, now going by the name Bruce, living with his family. Initially reluctant, Bruce is persuaded by his wife and children to join the rescue mission. The group then tracks down Clay at an abandoned Bergen miniature golf course, where they discover a hidden colony of Pop Trolls who fled during the Trollstice and have remained in hiding, unaware that the Bergens are no longer their enemies. The colony's leader, Viva, is revealed to be Poppy's long-lost sister. Although Poppy is thrilled at first, the reunion turns sour when Viva refuses to believe the Bergens have changed. Despite Viva's attempts to stop them, the group escapes with Clay's help, leaving Poppy and Viva heartbroken over their disagreement.
Meanwhile, Velvet and Veneer continue draining Floyd's essence. Velvet reveals that she forged a letter from Floyd to lure his brothers into a trap. She and Veneer's assistant, Crimp, develop shoulder-pad suits designed to extract essence more efficiently, planning to use them on the entire BroZone once they are captured.
The Trolls reach Mount Rageous but struggle to work together, failing to achieve the Perfect Family Harmony. Tensions rise when Branch accuses his brothers of still treating him like the baby of the group, revealing the loneliness he endured after their grandmother's tragic death. Branch, Poppy, and Tiny attempt to rescue Floyd on their own but are captured alongside John Dory, Clay, and Bruce during a chaotic showdown with Velvet and Veneer. The villains begin using the shoulder-pads to extract the brothers' essence, preparing for their grand performance.
Just as all hope seems lost, Viva arrives with unexpected reinforcements: Bridget and Gristle, who had stumbled upon the hidden golf course during their honeymoon. Encouraged by Poppy and Branch, the brothers finally put aside their egos and work together. Joined by Poppy and Viva, they perform the Perfect Family Harmony, breaking free from their prisons and destroying Floyd's bottle just in time to save him.
Veneer, having grown uneasy with Velvet's manipulations, publicly confesses their scheme. The siblings are arrested, ending their reign of chaos.
The Trolls return to Vacay Island, where BroZone performs a triumphant comeback concert. They are joined by Kismet, another band Branch was once part of, celebrating not just their reunion but the power of family and teamwork.
