The film starts with a flashback to when Branch (Alan Kim) was a baby. He performed with his brothers – John Dory (Eric Andre), Floyd (Troye Sivan), Spruce (Daveed Diggs), and Clay (Kid Cudi) – in the boy band BroZone under the name Bitty B. The boys are about to go up and perform, but Branch is nervous. Floyd, the brother that he is closest with, reassures him that he will be fine.
BroZone then goes up to do their song and attempt to achieve the perfect family harmony, which is powerful enough to shatter diamonds. John Dory’s showboating causes them to not only mess up the song but end up tangled together as they tumble off the stage. The brothers all fight, with John, Clay, and Spruce all angrily splitting up and going their separate ways. Floyd leaves Branch with their grandmother and promises to come back for him.
In the present, Branch (Justin Timberlake) holds animosity towards his brothers for abandoning him. Poppy (Anna Kendrick) catches him listening to BroZone, not realizing he used to be part of the group. They then go off to join the other Trolls and Bergens as King Gristle (Christopher Mintz-Plasse) and Bridget (Zooey Deschanel) are getting married (“Let’s Get Married”).
Before the wedding can continue, John Dory swoops in and approaches Branch. Poppy then learns of Branch’s boy band history, and John informs Branch that Floyd is in trouble. John received a message from him saying that he was trapped on Mount Rageous by brother-sister pop star duo Velvet and Veneer (Amy Schumer and Andrew Rannells). They are keeping him prisoner in a diamond perfume bottle that sprays them with his Troll essence to give them the ability to sing, since they are otherwise untalented hacks. John wants to round up the rest of his brothers to help save Floyd, and while Branch is apprehensive about seeing them again, he tells Poppy that he will do it for Floyd. Poppy then joins him on the adventure, and Gristle and Bridget are married by Miss Maxine (RuPaul).
Velvet and Veneer finish recording a music video (“Watch Me Work”) and doing an interview before going back to their put-upon assistant Crimp (Zosia Mamet). They go to take more of Floyd’s essence, but the more they use of him, the more it is starting to kill him. Veneer is conflicted about keeping Floyd prisoner, but he cannot let go of his newfound fame and wealth.
Poppy and Branch join John Dory in a giant bug that also operates as a bus named Rhonda, with Tiny Diamond (Kenan Thompson) stowing away. They go to Vacay Island, inhabited by a bunch of Muppet-looking creatures, to find Spruce, who is now going by “Bruce” since he has married a taller woman named Brandi (Patti Harrison) and has more than ten kids with her. Spruce is happy to see Branch again, but not John Dory, since he is the one most blamed for the band’s split. Spruce then joins his brothers in proving he was once in a boy band (“BroZone’s Back”) before joining the mission to save Floyd. Brandi supports his endeavor so she can take care of the kids, but says he has to do it himself for a month when he returns.
Crimp develops a device that is supposed to make smoothies and create an essence spray from fruits, but Velvet uses Floyd’s diamond prison with the machine to easily absorb his essence, to Crimp’s horror.
The Trolls then go through a weird trippy portal after Tiny presses a “hustle” button inside Rhonda, taking them to a secret golf course where Clay is located. They are met by another pink Troll named Viva (Camila Cabello), who has an aggressively cheery personality. It is quickly revealed that Viva is Poppy’s long lost sister since King Peppy (Walt Dohrn) is their dad. Poppy gets excited over learning she has a sister (“It Takes Two”). However, when the brothers tell Clay about their mission, Poppy and Branch separately learn that Viva got separated from her family when Poppy was a baby back when the Trolls and Bergens were enemies, and the Bergens attacked their village. When Poppy and the others try to leave, Viva tries to stop them since she and the other Trolls in her land still believe the Bergens are evil. Poppy tries to explain that that’s no longer the case, but Viva ends up not joining them.
Floyd pretends to be dead so that Velvet and Veneer will have to let him go, but as he tries to escape, Veneer catches him. Floyd tries to appeal to Veneer’s better nature, but he ultimately cannot stand up to Velvet and has to keep Floyd captive.
Clay joins the others as they make their way to Mount Rageous. They start to get into another fight because of John Dory, and when Branch learns that they plan to go their separate ways after saving Floyd, he blows up on his brothers and decides to be the one to walk out on them this time. Poppy goes after him and motivates him into forgiving his brothers and continuing to work with them.
Gristle and Bridget ride their motorcycle during their honeymoon and come across the golf course. Viva’s Troll tribe captures them and ties them up to throw them into a grinder. The two decide to spend their last moments making out, making the Trolls see they are not evil. Viva then recognizes Bridget from Poppy’s scrapbook about her and realizes she was telling the truth about Bergens. Viva unties Gristle and Bridget and joins them to go help Poppy and her friends.
The Trolls make it to Mount Rageous and try to sneak their way into Velvet and Veneer’s dressing room to get Floyd, but the villains capture John Dory, Clay, and Spruce and add them to their essence-stealing devices. They go off to perform their big show at the Rage Dome while Poppy, Branch, and Tiny find Crimp after Velvet and Veneer locked her in the closet. They use Rhonda to confront the duo in front of the crowd arriving for the show, telling everyone that they are frauds. Velvet and Veneer then lead the Trolls on a chase across Mount Rageous before Viva, Gristle, and Bridget arrive to help. They manage to free John, Clay, and Spruce, but Velvet tries to use whatever is left of Floyd’s essence to keep singing. The brothers, joined by Poppy, Viva, and their friends, then manage to achieve the perfect family harmony (“Better Place”) to break the diamond prisons. Floyd appears to lose all of his essence and seems dead, but he is okay moments later.
Veneer then becomes overwhelmed with guilt and confesses his and Velvet’s crimes to the entire crowd. Crimp places them under arrest, and the brothers all happily reunite, while Poppy and Branch finally kiss.
The Trolls return to their kingdom, with Viva reuniting with Peppy after years apart. Branch then reveals to the others that BroZone was not the only boy band he was part of, and brings out four other Trolls (Lance Bass, JC Chasez, Joey Fatone, and Chris Kirkpatrick) for essentially a Trolls *NSync reunion, leading to another big musical performance.