When Timmy realizes that everyone in Dimmsdale has a pet except for him, he adopts Sparky, a mischievous fairy dog. But owning a fairy dog proves to be a much bigger challenge than Timmy could have ever predicted. 
Timmy Turner is known as the worst pet owner ever from his previous pets. After realizing he is the only person in Dimmsdale without a pet (even Mr. Crocker has a pet parrot), he tries to get a dog. Wanda tries to talk him out of it, thinking it's a bad idea. His parents disagree to getting a dog, so he wishes for cats to appear in the kitchen making his mom, who is highly allergic to them, get sick. His plan was for his dad to ask Timmy to get a dog to chase the cats away.
His plan went kind of as expected, except that Mr. Turner thought Mrs. Turner was a monster because of her allergic reactions that made her swell up. As planned, Mr. Turner wants Timmy to get a dog to get rid of it. Timmy asks his godparents to poof him to a pet store. Timmy, Cosmo, Wanda, and Poof appear at the Fairy World Pet Store. Timmy asks why they went there, and Wanda says it's because he's banned from all the regular pet stores.
Timmy tells Jorgen, who was filling in for a family member of his, that he wants to get a dog. Timmy sees a cage with growling noises coming from it, and takes off the cover, only to reveal a dog named Sparky eating steak at an unusually small dinner table. Sparky immediately notices Timmy and races over to him and licks him. Timmy giggles and asks Jorgen if he can have him because he's super cute and friendly. Jorgen says that Sparky is a lot of work and has been brought back to the pet store more times than Cosmo. Timmy keeps asking and Jorgen agrees, but only because his shift is almost up. When they get back home, Sparky shows Timmy, Wanda, Cosmo, and Poof all the tricks he can do and amazes them.
Timmy and Sparky are then shown playing video games. Mr. Turner walks over to them, disguised as a lamp. He asks about the dog, and Timmy says his name is Sparky. Mr. Turner says the monster is in the kitchen, and is trying to make him lasagna. He tells Sparky to go chase the monster out of the house. Sparky jumps on his motorcycle and rides into the kitchen, chasing the cats and Mrs. Turner out of house. Mr. Turner says there's something weird about the dog, how it rides a motorcycle like a monkey, and the monster screams like a girl. Timmy says he's just a regular dog, and he tries to demonstrate. Timmy asks Sparky to go fetch the paper. Sparky runs off and comes back with the newspaper boy. Timmy corrects Sparky by saying that he wanted the newspaper. So, Sparky bites the newspaper from the newspaper boy's hand, and walks into the bathroom.
Timmy is seen upstairs, reading a book while Sparky plays with the 'chew toy' (Poof). Vicky bursts in, and says she was in a fight with her boyfriend Justin Jake Ashton, and she was going to take it out on Timmy. Vicky takes a chainsaw, and raises it in the air. Timmy screams in terror, and then Sparky jumps in front and growls. Then he bites her leg, making her yell in pain, making her drop the chainsaw, making a hole where she was standing. Then she falls into the basement. Sparky says he protected Timmy from her because he loves him, and that Vicky scared him, as well. Timmy says with tears in his eyes "You're the best dog ever..". Vicky then yells "There's a barrel of pushpins down here!". Sparky then licks Timmy. Wanda admits that it was a good idea to get Sparky. That night Timmy said that everyone was wrong about Sparky and that he couldn't see how he could be any trouble.
Timmy wakes up, only to see that his house was under the ground. Sparky says that fairy dogs don't sleep and he did some digging the night before. Timmy wishes the house to be above ground again. When he walks downstairs he sees that Sparky had messed up his house and dug a ton of huge holes outside, causing trouble. Timmy wishes for everything to be back to normal, but Cosmo and Wanda's wands were buried. Then Sparky starts scratching himself because he feels itchy when fleas start appearing on him. Apparently Timmy doesn't remember to take care of the dog by sprinkling magic flea powder on him. The fleas start flying off and biting humans in the city, causing them to become hairy dog-like creatures. Timmy tries to get Sparky to fetch the wands, but then he comes back with other items that resemble the word "wand" such as Wanda, a blonde, and his neighbor Juan. Some of the infected people come running up to Timmy and his fairies and Sparky. They run away, and the dog people fall into a hole that Sparky buried. They run up to Timmy's room and tell Cosmo, Wanda, and Poof to find their wands before things get any worse, but it turns out things have already gotten worse according to Chet Ubetcha, who says dog hair has begun to clog the city's drains and sewers, people are starting to turn into dogs, and also that a new dog grooming shop is in business.
A few moments later, a flea bites Chet Ubetcha and he turns into a dog and starts saying "This just in.." then starts barking the news and walks off screen. Meanwhile, Vicky is seen busting the door open with a chainsaw and starts barking by saying "twerp" over and over again. Vicky swings the chainsaw around and busts open the magic flea powder box, and some powder falls out and cures Vicky. Vicky is confused for a second, but then continues threatening Timmy. Sparky, once again, scares Vicky into dropping the chainsaw, making a hole around her, and she falls into the basement. Then she yells "I'm okay! A barrel of pushpins broke my fall!". Timmy tries to think of a way to spread the magic flea powder throughout the city to cure everything the same way it cured Vicky. Just then, Mr. Turner calls Timmy to come downstairs to check out his new crop duster. Timmy then makes a plan where his fairies go find their wands and Timmy and Sparky will take care of the magic flea powder. Timmy rushes downstairs and sees the crop duster made out of the pile of wood from what used to be their entertainment center. He called it the 'Dinkleduster', because he wants to drop dust over Dinkleberg's house because he's allergic to dust (he's also allergic to lobsters, but that's too expensive for Mr. Turner to buy). Timmy asks if he can use it to drop magic flea powder throughout the city, and Mr. Turner is okay with it, as long as he doesn't have to go along.
Timmy then flies off into the city, spreading magic flea powder everywhere. Not long after that, everyone was back to normal, and not dogs anymore. But then Timmy realizes he doesn't know how to fly a plane, then the plane starts descending. Timmy is pulling on the steering wheel and apparently he pulls a little bit too hard, because it snaps off from the force that Timmy has been putting on it. Then he says that his dad doesn't know how to build one either. Sparky says that he found a parachute, and says he'll test it, then give it to Timmy. Sparky jumps off the plane with the parachute and yells "It works!". Meanwhile, the plane is moving downward even faster, and the plane is breaking apart, leaving only the pilot's seat. Timmy is screaming while this all happens, and crashing through clouds and landing in the city's public pool also seen in the earlier episode Emotion Commotion!. Timmy finally admits that it was a bad idea to get a dog in the first place, and says that Sparky is too much trouble and a disaster. He says that he has two options: Take Sparky back to the pet store, or go on vacation forever. Sparky is seen in the background, landing on the ground safely with the parachute. When he heard what Timmy had said, he stared at the ground, depressed, and walked away.
Timmy is seen back in his room with his fairies, and notices chewed up furniture and paw prints everywhere. He sees a DVD on his bed labeled 'Play Me'. Timmy puts the DVD in the player, and starts watching the tape. Sparky appears on the screen, apologizing for being so much trouble. He says he's going away forever because Timmy doesn't want him anymore. He says the DVD consists of his favorite moments with him. A screen appears that says 'Timmy Turner - World's Best Pet Owner'. These moments are: Sparky driving the car, eating pasta with Timmy (Sparky is highly allergic to pasta, but ate it only because Timmy likes it), and Timmy and Sparky saving the city. They didn't actually do the saving the city part, but Sparky was hoping to do it with him, but apparently not now. Sparky then bursts into tears. Timmy has seen enough, and turns off the TV. Timmy then says he shouldn't have been so hard on Sparky, he was just being a dog. Timmy then wants Sparky back, but he doesn't know where he is. They hope he's not at the pound. Timmy says he's going to the pound. But then, Chet Ubetcha appears outside of Timmy's window, and says turn on the TV, because he's about to deliver breaking news, and he rushes to the station. Timmy turns on the TV, and Chet Ubetcha (almost out of breath) says that dog hair has completely clogged the city's storm drains, pipes everywhere are backing up, and the Dimmsdale Dam is about to burst, and flood the city. Chet Ubetcha realizes that his car is about to be towed, and rushes off.
Wanda tells Timmy that his house will be destroyed first, because the dam is in his backyard, caused during a week called "Timmy Turner's Week of Really Stupid Wishes". Timmy runs outside and sees Sparky at the bottom of the dam with a guitar. Sparky says he's leaving, but he couldn't leave without saying goodbye and a song. Sparky starts the song, but then he gets blasted by a wave of water. Timmy jumps into the water to save Sparky from drowning. He finds Sparky and drags him to a small patch of land and thinks he's too late, and admits he probably is the worst pet owner ever. But then Sparky spits a bunch of water out of his mouth, making Timmy fall back into the water. Then Sparky, like nothing had just happened, tries to continue his song, but realizes he doesn't have his guitar in his hands and stops. Timmy exclaims "Sparky, you're alive!" to him. Sparky says that fairy dogs are immortal and that Timmy was crazy to jump into the water. Timmy then hugs Sparky, and apologizes for giving up on him, and that he'll never do it again. Sparky then licks Timmy's face, and Timmy says he loves him too.
Meanwhile, Chet Ubetcha says that the city has been destroyed by the flood, but in less-depressing news, Timmy has saved his dog from the flood, and that it turns out that Timmy may not be the worst pet owner ever. Chet Ubetcha asks Timmy what his dog's name is, and Sparky proudly says "My name's Sparky". Chet Ubetcha comments on how he barks funny. Cosmo notices starfish playing with sticks in the water, but they're actually their wands!
Timmy wishes everything back to normal, then takes Sparky for a walk into the sunset.