"The Monster of Phineas-n-Ferbenstein" is a Halloween episode of the animated TV series Phineas and Ferb.
An American comedy-musical animated television series, Phineas and Ferb was created by Dan Povenmire and Jeff "Swampy" Marsh for the Disney Channel that originally previewed on August 17, 2007, though the series was officially premiered on February 1, 2008. The main plot of the show follows American boy Phineas Flynn and his English stepbrother, Ferb Fletcher, who are busy trying to make the most of their summer vacation. Each and every day, the boys plan out and initiate a plan to embark on a grand new adventure, much to the annoyance of their controlling sister, Candace, who seeks to punish her brothers' shenanigans by telling on them to their mother, Linda Flynn-Fletcher and occasionally their father, Lawrence Fletcher. Despite this, she always fails to get their parents to appear where she needs them to be while proof of the shenanigans still exist.
A secondary plot is also in effect, in which the boys' pet platypus Perry is secretly a spy by the codename of Agent P, working for the OWCA (the Organization Without a Cool Acronym), who fights a mad scientist named Dr. Heinz Doofenshmirtz.
Usually these two plots intersect at the end of each episode, leading to the erasure of any trace of the boys' project, leaving Candace very frustrated.
The boys learn about one of Ferb's Victorian ancestors, who helped a vaguely familiar scientist create a monster. Meanwhile, Doofenshmirtz recalls the story of his great-great-grandfather Dr. Jekyll Doofenshmirtz to Perry, as they both wait out their lockdown.