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Phineas and Ferb team up with the Mystery Twins to help Wander and Sylvia spread happiness across Gravity Falls against Lord Hater's newest scheme.
Phineas and Ferb travel to Gravity Falls as a kick start to their summer vacation and meet the Mystery Twins, Dipper and Mabel, as they cross each others paths when uncovering mysterious cryptic messages found across town
Meanwhile, Lord Hater, leader of the Hater Army who plans to spread hatred and power throughout the galaxy, schemes to exploit the supernatural side of Gravity Falls to further help his overall plans. Wander, an intergalactic nomadic traveler and his arch-nemesis, hears about this and goes over to Gravity Falls with his trusty sidekick, Sylvia, to warn the people and help spread happiness in the town against the hatred that's been planned to spread.
Phineas, Ferb, Dipper, and Mabel find out that the messages lead to a celestial battle against happiness and hatred, and just then, Wander and Sylvia meets the others and tells them about Lord Hater and the Hater Army. All six team up to spread happiness across Gravity Falls.
Meanwhile, Lord Hater employs and teams up with Dr. Doofenshmirtz, an evil scientist who right now has similar intentions in which he exploits the supernatural to his use of his scheme to rule the Tri-State Area, to invent an Inator that'll help them succeed with their goals.
Other subplots in this special include Perry the Platypus, Phineas and Ferb's pet platypus, also having to stop both Dr. Doofenshmirtz and Lord Hater while trying not to expose himself as a secret agent. Waddles, Mabel's pet pig, follows Perry which results into making Waddles a secret agent in training under Perry the Platypus and that Waddles also has to not expose himself as a secret agent. Dr. Doofenshmirtz and Peepers, Lord Hater's #1 henchman, start a rivalry between them to see who's the better assistant. Candace, Phineas and Ferb's older sister, becomes good friends with Wendy, a teenage girl who works part time at the Mystery Shack, in which Soos, another person who works at the Mystery Shack, becomes madly in love with the former girl. Grunkle Stan, the proprietor of the Mystery Shack, calls the aid of an old friend, Major Monogram, Perry the Platypus' commanding officer at the O.W.C.A., to help him with the Mystery Shack.
- Waddles from Gravity Falls becomes an agent for the O.W.C.A. under Major Monogram's division.
- This is the first crossover special for Phineas and Ferb, Gravity Falls, and Wander Over Yonder.
- This crossover is counted as a Phineas and Ferb special.
- It will be the show's fourth one-hour special after Phineas and Ferb Summer Belongs to You!, Phineas and Ferb: Mission Marvel, and Phineas and Ferb: Star Wars.
- The special uses a different animation style from Phineas and Ferb, Gravity Falls, and Wander Over Yonder so that characters from all three shows fit together.
- Vincent Martella as Phineas Flynn
- Thomas Sangster as Ferb Fletcher
- Jason Ritter as Dipper Pines
- Kristen Schaal as Mabel Pines
- Jack McBrayer as Wander
- April Winchell as Sylvia
- Ashley Tisdale as Candace Flynn
- Linda Cardellini as Wendy Corduroy
- Alex Hirsch as Grunkle Stan, Soos
- Dee Bradley Baker as Perry the Platypus, Waddles
- Dan Povenmire as Dr. Doofenshmirtz
- Keith Ferguson as Lord Hater
- Tom Kenny as Peepers
- Jeff "Swampy" Marsh as Major Monogram