- After one of Dr. Doofenshmirtz's inventions lands in the yard, the boys try to find out what it does through reverse engineering. / Phineas and Ferb and the neighborhood kids to on an epic journey to find the lost city of Atlantis.
- Atlantis:
Candace is looking for some peace and quiet on the beach. Unfortunately, Phineas, Ferb, Isabella, Buford, Baljeet and Irving are with her and Linda. Isabella asks everybody if anyone wants to play volleyball with her. Linda points out to Candace that there is a sandcastle contest on the other side of the beach. Linda tells Candace to join up but she replies saying that Phineas and Ferb would build a skyscraper or something huge so it wouldn't be worth to compete. Phineas joins in the conversation saying that they'll be finding Atlantis, which was probably off the coast of Danville. Isabella and Buford agree with the idea and Irving comments how he was lucky to be with them.
Perry about to jump in the guitar case. Perry walks up to a man playing a guitar (a variation of Perry's theme tune) and Perry jumps into the man's guitar case. He falls into his chair where Carl greets him, wearing a mustache and pretending to be Major Monogram. Carl tells Perry half of his mission when the actual Major Monogram appears and takes over the briefing. He concludes the briefing and Perry flies off to Heinz Doofenshmirtz's blimp. The kids are in scuba diving gear and Phineas tells Linda that they're going to find Atlantis. Linda says that they have such active imagination. Candace realizes that the group leaving the beach means they won't participate in the sandcastle competition. Candace grabs Linda and tells her they will be entering the competition after all. Shortly they both start building a sand castle.
The group is underwater when they start to find some lost artifacts, such as the Titanic, Amelia Earhart's wrecked plane and the tomb of Poseidon. Phineas tells the group to focus on finding Atlantis and Ferb's GPS gives the group directions. Baljeet then asks Buford why the water is always warm around him which Buford simply returns a glance at him. Back at the beach Candace orders Linda what they need to do in order to build a successful castle.
Phineas and the gang in Atlantis. Meanwhile, Doofenshmirtz is in his blimp watering the plants he bought when quietly Perry climbs through the window and lands in a sack labeled Fertilizer. Doofenshmirtz continues to water the plants whilst telling Perry what he is about to do. He has invented a growth formula to make the plants evil and help him take over the Tri-State Area.
As the group gets closer to Atlantis Irving decides to take photos. Once he has finished taking a picture, an eel steals his camera. Irving complains but Phineas tells him to use his other camera; his mind. Irving tries it whilst Isabella finds a seahorse. Baljeet says she can now earn the Underwater Equestrian Patch and Isabella says that she was being sarcastic; seahorses aren't big enough to ride. Buford then scoops a bunch of seahorses and says he is able to ride them. Phineas finds a door with ancient inscriptions telling him how to open the door. He starts to read it when Buford decides it would be easier to punch the door. A huge rock separates, showing the lost city of Atlantis. The group explore Atlantis whilst singing a song about it. Once they have finished exploring, Irving decides to take a picture of the group in Atlantis in his mind and Buford comments how he took the picture thing to a new level. Phineas suggests bringing Atlantis above the water for everyone to see.
Back on the beach, Linda and Candace are still trying to make a sandcastle but they fail for the third time. Candace grows impatient and mentions how good she used to be building sandcastles. Linda replies to Candace just to relax and have fun when Jeremy walks over and sees Candace and Linda and explains he's the judge. He then says he can't wait to see the Flynn-Fletcher sandcastle. After he leaves Candace renews her effort to finish the sandcastle and further orders Linda around.
Meanwhile in the blimp, Doofenshmirtz finishes spraying the plants with something that appears to be smoke. The plants come to life and start growing and turning evil. Doofenshmirtz's plan to attack Perry backfires on him badly with the plants attacking both him and Perry.
Back in Atlantis a delivery guy drops an order and asks Phineas if he isn't too young to be discovering Atlantis to which Phineas replies yes they are. The delivery guy commends them and takes of in his van. Then Phineas tells the group to start working.
After that we see that Doofenshmirtz is almost covered by the vines of the evil plants he created and asks Perry for help, to which Perry frees himself and jumps off the blimp leaving Doofenshmirtz alone.
Candace and Linda start working very hard to get a good sandcastle but most of it collapses, again. Jeremy walks over and starts to judge their sandcastle when Atlantis floats up. It is being lifted by four weather balloons attached earlier by Phineas, Ferb, and friends. Candace is explaining about how their castle isn't perfect but how they enjoyed making it when Jeremy sees Atlantis, believing to be Candace's sandcastle and decides to award them the first prize. He says he is going to get their blue ribbon and walks away. Linda tells Candace it must have been the way he feels for Candace that he gave them the blue ribbon.
Back with Doofenshmirtz, the plants take over most of the blimp. Perry returns a few seconds later with a chainsaw and starts cutting all of the plants and freeing Doofenshmirtz. The spiked plants thrown overboard hit the balloons that support Atlantis and causes Atlantis to sink again. Atlantis sinks before Candace and Linda see it as they think Jeremy meant their half-collapsed castle.
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