The movie starts with Cloud Guy (Walt Dohrn) recapping how Poppy (Anna Kendrick) and Branch (Justin Timberlake) saved their world. What they don’t know yet is that there are other Troll islands besides theirs. We cut to the land of Techno Trolls, where King Trollex (Anthony Ramos) is leading a party to “One More Time”. The celebration is interrupted by the arrival of the Rock Trolls, led by Queen Barb (Rachel Bloom). She demands the string of the Techno Trolls, and when Trollex refuses to give it up, Barb plays a powerful rendition of “Rock You Like a Hurricane”, bringing destruction to the land. Trollex has no choice and gives up the string before she kidnaps him and the rest of the trolls.
On the Pop Troll island, Poppy is still having fun with her friends. Branch is struggling to tell Poppy how he really feels about her. The group gathers when Guy Diamond (Kunal Nayyar) is about to have a baby. He gives birth to Tiny Diamond (Kenan Thompson), who raps his introduction. After a whole song-and-dance session, a Rock Troll bat creature flies in with a message from Barb to Poppy, saying she would like her to bring her string for her Rock World Tour. King Peppy (also Walt Dohrn) panics and tries to cover things up until he’s forced to reveal to Poppy and the other Trolls that there are other Troll Islands. Long ago, the Troll ancestors came up with six different music groups – Pop, Rock, Techno, Classical, Funk, and Country. While they were all united, the Trolls started growing intolerant of each other’s music before the leaders of each group decided to take their strings and go their separate ways. Peppy and Branch are ready to hide all the other Trolls, but Poppy wants to take a chance and meet with Barb, despite the protests of the others.
That night, Poppy sneaks out to go on her journey on a hot air balloon, when she is caught by Branch. Although she appears set to go off on her own, Branch reluctantly joins her. Meanwhile, Cooper (Ron Funches) sees the book of the Troll ancestors and sees that one of them is a giraffe Troll like him. He sets off on his own to find his true family.
Barb returns to Rock Troll Island
Barb returns to Rock Troll Island where she has a second string to add to her powerful guitar, with which she will use to unite the whole world…under ROCK. Her father is King Thrash (Ozzy Osbourne), who isn’t all quite there, but is still a firm believer in the power of rock.
On their ride over, Poppy and Branch find that Biggie (James Corden) and Mr. Dinkles (Kevin Michael Richardson) have stowed away after eating all the cotton candy that Poppy was saving for Barb. They arrive on the Classical Troll Island town of Symphonyville, which has been devastated by Barb and the Rock Trolls. The group finds Pennywhistle (Charlyne Yi), who tells Poppy that Barb came and overwhelmed their leader, Trollzart (Gustavo Dudamel), dismissing classical music as boring before stealing the Classical string and kidnapping Trollzart and the Classical Trolls. Poppy realizes Barb’s true intentions and decides she has to save the other Trolls before Barb destroys their world. Although Branch wants to go back, Poppy insists on continuing the journey, even having brought the Pop string with her. She pinky-promises Biggie that she will protect them, and a pinky promise for Trolls is powerful enough to send waves across the land. Pennywhistle stays behind the help rebuild the land.
Barb receives Poppy’s cute invitation (with glitter) to join forces and become best friends, which she takes as an insult. She decides to send out four sets of bounty hunters to go after her – Chaz, the Smooth Jazz Troll (Jamie Dornan), the Reggaeton Trolls led by Tresillo (J Balvin), the K-Pop gang (Red Velvet), and the Yodelers (who are absent). Barb promises them that if they capture Poppy, she will let them keep their own music on a small piece of land.
Poppy, Branch, and Biggie (and Mr. Dinkles) make it to the Country Troll Land town of Lonesome Flats, led by Delta Dawn (Kelly Clarkson). The Country Trolls sing a gloomy song, which Branch likes but Poppy thinks it means that they need to introduce them to the music they love. They do a medley of feel-good songs (“Wannabe,” “Who Let The Dogs Out,” “Gangnam Style”, and “Party Rock Anthem”), only to get thrown in jail. Delta tells them that they know about Barb and they are fine sticking to their own music without interference. As Branch starts to dig them out, they are freed by another Country Troll named Hickory (Sam Rockwell). He leads them on an escape, but Delta chases after them with her group before the heroes end up falling off a cliff into a river.
Hickory builds the group a raft to keep them going. Branch tells Poppy he doesn’t trust Hickory, but she thinks it just means he doesn’t trust her. They are then found by Chaz, who uses his Smooth Jazz powers to put Poppy, Branch, and Biggie in a trance. However, Hickory manages to block it out by putting gumdrops in his ears and then kicks Chaz into the river. Biggie freaks out and gets mad at Poppy for not being able to protect them, and he rides Mr. Dinkles back to the Pop Troll Island.
Cooper continues on his own journey, but he has been traveling for so long that he has started to hallucinate. Just when it looks like he’s ready to give up, an aircraft arrives and pulls him up inside.
The aircraft then finds Poppy, Branch, and Hickory, bringing them inside. It belongs to the Funk Trolls, where it is revealed that Cooper is a prince who was separated from his family when he was just an egg. His parents are King Quincy (George Clinton) and Queen Essence (Mary J. Blige), and his twin brother is Prince D (Anderson .Paak). When Poppy asks Quincy and Essence for their help in stopping Barb, they decline. Prince D tells Poppy through song the truth about what happened with the strings – it was the Pop Trolls who tried to take all the strings and tried to claim all of music as their own by remixing other types of music. The other Troll leaders grabbed their own strings and fled. Poppy tries to argue that differences in music and taste don’t matter, but the others think denying their differences means denying who they are. The Rock Trolls then start to attack, so the Funk Trolls try to fight back. Poppy, Branch, and Hickory are then ejected from the ship as Barb and her minions take over.
Poppy and Branch then argue as she is insistent on trying to continue the journey, but Branch tells her that she has been ignoring everyone who has been trying to tell her not to pursue this any further, and that a good queen would listen. Branch then goes off on his own, leaving Poppy behind. This leads them to sing on their own “Perfect For Me” as they quickly miss each other. Branch is then found by both the K-Pop Gang and the Reggaeton Trolls, who engage in a dance-off. Branch then convinces them that they don’t need to let Barb decide which music gets to be saved, and they decide to listen to him.
Poppy is found by Hickory
Poppy is found by Hickory, who reveals himself to have a partner, Dickory (Flula Borg). They are actually the Yodeler bounty hunters sent by Barb to get Poppy’s string, but Hickory actually feels bad about it. Barb then finds them and gets her hands on the string, now finally completing her mission.
Biggie and Mr. Dinkles return to their home, only to find that the Rock Trolls got there first. Guy, Tiny, Satin and Chenille (Caroline Hjelt and Aino Jawo), Smidge (also Walt Dohrn), and Legsly (Ester Dean) come out of hiding. Biggie feels bad for abandoning Poppy and resolves to round up his friends to go rescue her by using Rock Trolls disguises.
Barb keeps Poppy as a prisoner and reveals the pressure she feels to be a queen that everyone can follow, which Poppy can relate to. Her friends manage to sneak in as Barb’s back-up players. She presents the captured Troll leaders to the whole Rock Troll audience. Barb starts to perform “Barracuda” as she gets all the strings together to create the Ultimate Power Chord. Before she can unleash its true power, Branch and the other bounty hunters show up. Barb almost zaps Poppy with the Power Chord’s…power, before Branch jumps in and gets hit by it and turns into a Rock Zombie. She does the same to Trollex, Delta, Trollzart, Quincy and Essence before trying to hit Poppy with the Chord. She appears to become a Rock Zombie, and Barb tosses her the guitar to play rock until she reveals that she covered her ears with gumdrops. Poppy smashes the guitar, which destroys the strings and saps all the Trolls of their color. Barb blames Poppy for destroying all music, but Cooper grabs the mic and plays the music beating from within him. All the other Trolls from every clan start to join in, creating their own rhythm for the song “Just Sing”. Even Barb joins in, and all the Trolls make peace with the fact that they share different tunes. Poppy and Branch then express their love for one another.
Poppy and Branch later tell all the Troll children the new history of all Trolls and how they can appreciate each other’s differences.