The film begins when Toodee says "hello" to the kids. She tells them tomorrow is DJ Junior's birthday. She also tells them to keep a surprise from him because everyone is putting up a party. Meanwhile, Plex tells Brooklyn Borgquist and Leah to take DJ Lance's son somewhere, that way DJ Junior won't see what the Gabba gang are doing. When DJ Junior comes, Brooklyn and Leah tell him that they have to go somewhere.
DJ Junior asks Brooklyn and Leah where they are going. They tell him they are going to Universal Pictures movies to watch a film: "The Adventures of Rocky and Bullwinkle." And so they get ready and leave. After that, Muno tells his friends the coast is clear. So DJ Lance, Princess Winnie, Plex, Toodee, Brobee, Foofa and Muno can put up a party.
Meanwhile, at High Mountain, an evil guy named Huxley and his two sidekicks, Sid and Sam, are planning to kill Brooklyn, DJ Junior and Leah and make them not exist in the current timeline. Huxley tells Sid and Sam when Brooklyn, DJ Junior and Leah are done seeing the movies, Sid and Sam will sack them and carry them to High Mountain, making sure nothing can stand in their way. And so Sid and Sam go off.
Later, after seeing Rocky and Bullwinkle, Brooklyn, DJ Junior and Leah get out of the lobby and have a great time. Sid and Sam jump out and sack them. Back at Gabba Land, when DJ Lance, Winnie and the Gabba gang finish the party for DJ Junior, they hear a radar call that Brooklyn, DJ Junior and Leah are being captured and erased from existence by Sid and Sam after seeing the movie. So the Gabba gang decide they need to save them.
Big Bird, Elmo, Maria and Leela from Sesame Street come to help. They tell the Gabba gang that Big Bird already knows Sid and Sam are working for Huxley. What Elmo knows is that he and Big Bird can take them to the High Mountain while Maria and Leela are helping DJ Lance and Winnie with the party.
During that, Big Bird, Elmo and the Gabba gang meet Archie (voiced by Lance Henriksen) the lion, his wife Zinny (voiced by Cameron Richardson) and their playful son Nuka (voiced by Dave Thompson) who tell them two directions, so they also tell them to split up with a song: "Take The First Step." Brobee, Muno and Big Bird go to the left and Plex, Toodee, Foofa and Elmo go to the right. Suddenly, Big Bird, Brobee and Muno meet Rip Van Winkle (Steve Carell) who is sleeping next to a nearby tree. They shout at him to wake up and he did. Rip gives Brobee, Muno and Big Bird a clue to go to Brobee's Grandma's (voiced by Aubrey Plaza) house to ask her if they can borrow a map and they did.
Back at Gabba Land, DJ Junior's high school girlfriend, Sandy, her little brother, Jerry (Alexander Pollock) and their parents come to see DJ Junior, but DJ Lance warns them that DJ Junior, Leah and Katie have been captured by Huxley, Sid and Sam and that Big Bird, Elmo and the Gabba gang are going on an adventure to rescue them and Sandy understands that.
Meanwhile, when Plex, Toodee, Foofa and Elmo are walking around, Huxley's bugs trap them in the net, but they are rescued by a beautiful FBI agent, Maya Camelot, who fights the bugs and scares them away. Plex, Toodee, Foofa and Elmo thank Maya for saving them. She tells them that she was in the film Katie, DJ Junior and Leah saw before they got captured. Karen whistles for two carpet rides. Plex, Foofa and Elmo ride in one and Toodee and Karen ride in another one. Then Karen also tells them there are two more for Brobee, Muno and Big Bird if they see them.
Back at High Mountain, Huxley, Sid and Sam trap Katie, DJ Junior and Leah in a chair. DJ Junior informs Huxley that today is his birthday and he asks him why they are doing this to him, Katie and Leah. Huxley says that he wants to kill them, but Katie demands Huxley to let her, DJ Junior and Leah go. Huxley refuses and then tells Sid and Sam to bring the helmet control on Katie, DJ Junior and Leah and begin to kill in five hours.
When Brobee, Muno and Big Bird come to Brobee's Grandma's house, he asks her for a map. Brobee's Grandma tells them that Huxley's rare is at High Mountain. Then she reminds Brobee, Muno and Big Bird that Huxley's plan is to kill Brooklyn, DJ Junior and Leah and Brobee and Muno don't realize that Brooklyn is going to be killed on her second birthday. After that, when Brobee, Muno and Big Bird get out of Brobee's Grandma's house, they see Plex, Toodee, Foofa and Elmo with Maya Camelot on a magic carpet ride. Plex tells them there are two more for them. Brobee and Muno ride in one and Big Bird rides in the other. Elmo gets off the first one where Plex and Foofa are on and rides with Big Bird, but Brobee asks Maya why they are taking the carpet ride. Maya, Elmo and Big Bird tell him it is a long way with a song: "Ain't No Road Too Long."
Meanwhile, Big Bird, Elmo, Maya and the Gabba gang make their way to High Mountain. They form a plan for Plex, Brobee, Foofa and Muno to go to the left while Toodee, Big Bird, Elmo and Maya go to the right. While on the right, they don't realize that they have been captured by Sid and Sam. They bring Toodee, Big Bird and Elmo inside to destroy Brooklyn, DJ Junior and Leah wearing helmet control to be killed in fifteen minutes. Karen demands Huxley to let Brooklyn, DJ Junior and Leah go, but Huxley refuses in his greedy tone.
Back at Gabba Land, DJ Lance, Winnie, Maria and Leela invite Sandy, Jerry, their parents, Leslie Hall, Katie's parents and Scooder and his parents to DJ Junior's party. Then, suddenly, they hear Elmo, Big Bird, Karen and the Gabba gang are also beginning to be captured by Huxley, Sid and Sam and also hear that Katie, DJ Junior and Leah are beginning to be killed in ten minutes. So DJ Lance tells Maria and Leela to come with him to rescue Brooklyn, DJ Junior, Leah, Big Bird, Elmo, Maya and the Gabba gang while Winnie stays with the others for the party.
Meanwhile, back at High Mountain, Plex, Brobee, Foofa and Muno are walking around the hallway to the left when they hear Toodee, Brooklyn, Big Bird and Elmo screaming and getting captured by Huxley, Sid and Sam. Plex, Brobee, Foofa and Muno run to Huxley's room. When they get there, they see Big Bird and Elmo are in a cage, Toodee and Maya tied up side down and DJ Junior, Brooklyn and Leah are about to be killed and become non-existent in seven minutes. Huxley, Sid and Sam also capture Plex, Brobee, Foofa and Muno.
But outside the rare, DJ Lance, Maria and Leela make it to High Mountain. Maria calls all her friends from Sesame Street to help and they did come. Gordon, Chris, Cookie Monster, Super Grover, Bert and Ernie, Abby Cadabby and Oscar The Grouch come to help when DJ Lance, Maria and Leela save Big Bird, Elmo, Maya and the Gabba gang.
After Huxley, Sid and Sam tie up Plex, Brobee, Foofa and Muno, DJ Lance and the Sesame Street gang barge in and Ernie demands Huxley, Sid and Sam to let DJ Junior, Katie, Leah, Big Bird, Elmo, Karen and the Gabba gang go. But Katie, DJ Junior and Leah are about to be killed in five minutes. So then Super Grover uses his super powers to destroy the helmet control and it knocks Katie, DJ Junior and Leah unconscious. DJ Lance then uses his power to destroy Huxley, Sid and Sam.
After DJ Lance has destroyed Huxley, Sid and Sam, Maria, Leela, Gordon and Chris free Big Bird, Elmo, Maya and the Gabba gang, but then they see Brooklyn, DJ Junior and Leah dead and about to disappear forever. To wake them up, Abby uses her magic wand to heal them and Brooklyn, DJ Junior and Leah wake up. DJ Junior hugs his dad so much and DJ Lance loves his son too.
Plex then whispers to Maya and the Sesame Street gang, telling them to come to DJ Junior's birthday party in Gabba Land and they would love to. So DJ Lance, Brooklyn, DJ Junior, Leah, Karen, the Sesame Street gang and the Gabba gang go back home to see Winnie, her baby daughter, Princess Robyn, Sandy, Jerry, their parents, Scooder, his parents, Leslie, Brooklyn's parents and the others for the party.
When DJ Lance and the others finally get home, everybody surprises DJ Junior and Winnie makes him a birthday cake. DJ Junior blows out a candle. After that, the Gabba residents and the Sesame Street members perform a new song: "Can You Tell Me How To Get To Sesame Street". Then Sandy gives DJ Junior a birthday kiss and they begin to fall in love. During the credits, the songs begin with "Precious Wings" and "One Little Star".