Barney's old friend Captain Neweyes (Walter Cronkite), and his alien pal Vorb (Jay Leno), find his brother Rex (John Goodman) and three other dinosaurs give them his special Brain Grain cereal, which makes them smart and intelligent, and take them to present day New York City during Thanksgiving Day, where they'll meet children who want to meet them back, with help by Barney, BJ, Baby Bop, Riff and of course, the Backyard Gang, (accompanied today by Holly Hobbie), and some help from Twilight Sparkle and Spike. When they get there, they meet a lonely boy named Louie, who plans to run away and joining the circus. While Barney tries stop him, the others help him get there. Along the way, they meet another kid: a girl named Cecilia, who soon falls in love with Louie, and then they all run into trouble when they come across the circus ringmaster, and Neweyes' brother, Professor Screweyes (Kenneth Mars), who plans on using the dinosaurs, including Barney and the little ones, for evil by turning their brains off with his Brain Drain pills, which turns them back into violent monsters.
- Dipper Pines
- Ferb Fletcher
- Holly Hobbie (guest)
- Isabella Garcia Shapiro
- Mabel Pines
- Phineas Flynn
- Vanellope von Schweetz
- BJ, Baby Bop, and Riff will join Barney and friends in this adventure.
- Twilight Sparkle, Spike, and Doodles appear in this movie.
- The four dinosaurs reveal to be old friends and frenemies with Barney, while Woog is revealed to also be BJ and Baby Bop's uncle.
- Both Phineas and Ferb and Gravity Falls were produced by Disney Channel.
- Big Bird makes a cameo in this movie.
- Rex will see Barney and the gang again in Barney Joins The Smurfs.
- Rex, Woog, Elsa, and Dweeb will reveal how they met Barney in Barney's Adventures of The Good Dinosaur. Also at one point, Arlo is mentioned.