Phineas and Ferb are in their backyard using a giant drill to tunnel to the center of the earth. Ferb hears a loud bang in the hole and Phineas wonders what happened. Phineas and Ferb goe down the hole with flashlights. Before long they see a huge metal box at the bottom, a curious Phineas pushes the top off and sees a huge green unconscious figure Phineas also sees an old letter. He reads it and discovers the figure was the platypus monster created by Phineas and Ferb's ancestors, Dr. Phineastein and Ferbgor, who put the monster in suspended animation before they died. Phineas decides to revive the monster so they could play with him. Back on the surface, Perry talks with Major Monogram, and discovers Dr. Doofensmirtz has been buying his usual strange items, this time a cofee machine, some unknown chemicals, and a bag of sugar. Perry goes to investigate Doofenshmirtz hideout. Phineas and Ferb take the platypus monster to the basement and build electical towers to bring him back to life. when the monster is revived he instantly grows an attachment to Phineas and Ferb due to their resemblense to his old masters. Candace comes in to see whats going on. When she sees the monster she briefly goes away to tell mom, then she comes back to ask where she is. Phineas says she's at the mall, and Candace runs off. Perry arrives at Doofensmirtz' lair, he says this time he doesn't have to worry about trapping him because he's revived the grand invention made by his ancestor Dr. Jekyll Doofenshmirtz, the Concoction Brew-inator, a machine that can produce a potion that can turn a normal man into a monster. Doofensmirtz drinks the potion and becomes stronger, and is easily able to defeat Perry, afterwhich Doofensmirtz goes on a rampage through the town. Perry is to weak to get up and cannot stop him. Meanwhile Phineas and Ferb begin training the platypus monster, when Doofensmirtz bursts through the fence, Phineas thinks that Doofensmitz is just another monster from the past, and Doofensmirtz thinks that the monster is actually Perry effected by the Brew-inator, and the monster recongnizes Doofensmirtz from the past and chases him. Phines suggest they go help him, but Ferb says they should fix the fence first. Candace finds her mother at the mall and takes her back home. Phineas and Ferb finish with the fence, just in time for Mom and Candace to see, Mom thinks the boys have just been painting the fence. Mom goes into the kitchen to make lunch. Phineas, Ferb and Candace see a breaking news report, saying that the platypus monster and Dr. Doofensmirtz are fighting downtown. Candace agrees to help the monster, because she still has to show it to Mom. The boys and Candace gear up for the fight and head downtown with their bikes. The monster fight goes all the way to a bridge whenthe kids arrive. Doofensmirtz thinks they are backup from the agency. The fight resumes, it ends when Doofensmirtz pounds the ground and the right half of the bridge fell, causing Phineas, Ferb and Candace to dangle for their lives. The platypus monster and Doofensmirtz are safe on the left half. When Doofensmirtz was about to attack, he turned back normal, but it turns out he brought another sample of the potion with him. He drank it but this time turned into a fairy. The monster blows a huge gust of wind and Doofensmirtz is blown away. Meanwhile Perry finally gets up and is headed for the fight, when he sees Doofenshmirtz in the air, he shrugs and goes back to pet position. The monster notices the kids hanging and manages to rescue them only to fall into the river himself. The kids are touched, even Candace. Phineas says he's done so much for them, he deserves his own life. Then Perry arrives. During the credits the river takes the platypus monster to a forest, and finds a colony of platypuses that welcome him to their home, the monster looks back to the city and remembers Dr. Phineastein and Ferbgor burying him and saying they hope he finds his destiny in the future.