Phineas and Ferb lasted seven years, 2007 to 2015, for 130 episodes the show mostly kept to one story line. Two brothers build a huge contraption, their pets tries to stop a evil scientist, and the sister tries to bust the brothers. This formula is used in every episode, yet the show is still creative and entertaining. Phineas and Ferb isn't about the question, but the answer.
The plot and situation the charaters are in, are only foot notes, Phineas and Ferb isn't about the problem it's about the solution. How the characters come to that solution doesn't change either, the solution is the only non-consistent thing in the show. The problem and answer are usally revealed for the A plot before the quarter mark of an episode, leaving the rest of the A plot to be about using and attempting the solution. Phineas and Ferbs contraptions are creative and leave room for imaginative humor and characters.
The B plot is about a character coming to a solution, Doofenshmirtz tells the viewer and Perry how he got to his solution though flashbacks, every episode Perry then destroys his machine or solution. The only non-consistent things in this plot are the solution and coming to the solution. The B plot is a little less consistent then the A plot, things like setting and character change for a single episode more often. The B plot usually resets the episode at the end of it.
In the end Doofenshmirtz machine destroys Phineas and Ferb's contraptions, the boys older sister, the show goes back to its original status, so the next episode can start fresh.
Most of the episodes use this similar formula, excluding few episodes. I wouldn't call this show consistent each episode is vastly different from each other,the only thing that is consistent is the question. Phineas and Ferb isn't about why you do something, it's about what you do that will make your day special.
