Phineas and Ferb turn the city of Danville into a giant thank you card for Santa Claus because they feel nobody ever thanks him for all the joy he brings to the world. Doofenshmirtz uses a device called the "Naughty-inator" to put the city of Danville on Santa's naughty list.

It's the summer solstice, so Phineas, Ferb, and their friends decide to travel around the world in one day in an effort to create the "biggest, longest, funnest summer day of all time". But not everyone believes they can actually achieve what they have set out to do, especially Buford. Candace tags along because Jeremy is on a trip to Paris, and she fears that there he may meet other girls. Also, Isabella finds her affections for Phineas sorely tested when his steadfast focus on achieving his goal puts pressure on her. Meanwhile, Dr. Doofenshmirtz takes a father trip to Tokyo, Japan with Vanessa, but sets out with Perry and Major Monogram to find her when she joins Phineas, Ferb and the others.

Phineas and Ferb get trapped in an alternate dimension where the evil Doofenshmirtz rules the tri-state area. They must find a way back home with the help of their pet platypus named Perry, who they discover is a secret agent.

When Sylvester hears that a children's museum's prize exhibit is Tweety Pie, he tires to raid the museum in order to have lunch. He doesn't succeed in his mission, due to Tweety's smartness, and the children.

When the Boogey Man causes Grim to lose his powers, rank, and title in the Underworld Court for misusing his powers, he plots to seek out Horror's Hand and take over the world. It's up to Billy, Mandy, Irwin, and Grim to get to Horror's Hand before the Boogey Man does.

Bugs can't seem to lose at Sam's casino. Then Sam realizes that Bugs has been cheating -- he has rabbit's feet.

Phineas and Ferb travel across the galaxy to rescue their older sister Candace, who has been abducted by aliens and taken to a utopia in a far-off planet, free of her pesky little brothers.