Dog Man 4 starts out with Lil Petey at Dog Man’s house calling everyone down to the living room to come watch the Fair Fairy show and Dog Man and 80-HD, a robot Petey made to serve Lil Petey that Lil Petey takes everywhere, come down and the three of them watch the show together but after Fair Fairy has a violent episode, Dog Man quickly switches to another show called Reddy and Lil Petey asks why and Dog Man uses ASL to explain that Fair Fairy isn’t suitable for children and Lil Petey says ok. The next day, Dog Man drops off Lil Petey and 80-HD at Petey’s place but Lil Petey tells 80-HD to stay with Dog Man since he doesn’t want Dog Man to be alone when he’s gone. 80-HD salutes him and gets back in the car and after Dog Man howls at Lil Petey, Lil Petey goes inside where the hideout has experienced an intense makeover to make it more family friendly. Lil Petey calls for Petey and Petey comes to greet him and gives him a hug and leads him to the kitchen where he’s made scrambled eggs for the two of them. Meanwhile, Dog Man arrives at the police station and 80-HD drives home.
Once at the police station, Chief tells him to come to his office and Maude and the male mean cop (I’m just going to call him Bob) start snickering and Milly, who’s sitting at a desk across from Maude, shuts them down by telling him that Dog Man has saved the city three times while Maude and Bob were just pigging out on donuts. Bob and Maude start crying and Dog Man smiles at Milly, who winks back at him. Then, Dog Man goes into Chief’s office and Chief tells him that he needs Dog Man to go to a joint event that the Ohkay City Science Center and the government are hosting based on findings they’ve made after studying Flippy’s brain. Dog Man is scared because he has a little bit of social anxiety and Chief says Sarah will be there too and he blushes after saying this. Dog Man gives him a “bruh” look and then leaves to go to the event.
Meanwhile, Lil Petey and Petey are playing a board game and Lil Petey asks if they have any other family in the area and Petey quickly says no and redirects the conversation to talk about robotics which Lil Petey is intrigued by. Meanwhile, Dog Man arrives at the event and meets up with Sarah who is so excited because her brother is going to be one of the speakers and she and Chief are getting married soon. Dog Man is confused and signs asking if they already had an engagement that he wasn’t invited to and Sarah says no because they’re going to skip getting engaged and just head straight into marriage and she promises that they would never do anything without him which cheers Dog Man up. Then, the lights turn off and Hudson says that the event will be starting and Sarah smiles and gets giddy and Hudson starts talking about their findings. After talking for some time, Hudson steps back and a scientist named Dr. Dookie comes and opens the floor for questions.
Sarah asks if it was a smart decision to study Flippy and Dr. Dookie claps back and says if it wasn’t a smart decision to study Flippy then it wasn’t a smart decision for Sarah to become a news reporter. Then, the whole thing turns into an argument between Hudson (defending Sarah), Sarah, and Dr. Dookie. Eventually, Dr. Dookie gets fed up, shuts down the event, and tells everyone to go home. Everyone leaves angry and the FLEAS, who have been watching the whole thing from the hideout, get an idea and Piggy says they should go and steal the dead Flippy and Bub and Crunky ask how because they’re all microscopic and Piggy says that he’s made a gadget to where they can fluctuate their sizes. Crunky and Bub get excited and Piggy pulls out three belts and as he puts his on, he explains that to activate them, you press the belt buckle and Crunky and Bub put theirs on and immediately start goofing around with them and Piggy gets mad and says that he’ll take the belts away if they don’t stop goofing around with them and Crunky and Bub apologize.
Then, Piggy says it’s their time to strike and the three of them leave. Meanwhile, at Petey’s house, Petey is sleeping while Lil Petey looks online and sees that there’s this dude named Mr. the Cat living in a nearby Schmokay Ville and Lil Petey gets in touch with him and invites him to their house. The next day, Sam Hamilton calls Fair Fairy to his office and he fires her because her show has 0% on Rotten Tomatoes and Fair Fairy responds by throwing a chair at him and then she storms off. Meanwhile, Petey and Lil Petey are having breakfast and the news is on as background noise saying how Flippy has gone missing and when Petey’s done eating, he asks Lil Petey what he wants to do today but before Lil Petey can answer, the two of them hear a knock on the door. Petey opens it and sees Grampa and Petey slams the door on his face and Lil Petey asks why he did that and Petey says because Grampa is a deadbeat who inflicted all kinds of pain upon him and his mom.
Lil Petey says that maybe Grampa has changed but Petey says that psychopath could never change. Lil Petey then says holding onto anger is like drinking poison and expecting the other person to die and pleads with Petey to give Grampa a second chance and Petey sighs and agrees to let him in. Petey begrudgingly lets Grampa in and Grampa pays him no mind and Lil Petey runs up to hug him but Grampa shoves him out of the way and heads for the TV where he turns on a soap opera while making crude remarks about Petey and Lil Petey. He says Petey should’ve just cut off his tail entirely because of how ugly it looks and he calls Lil Petey a runt. Lil Petey starts crying and Petey punches Grampa before absorbing Lil Petey into a hug.
Meanwhile, Chief calls Dog Man to his office because Dr. Dookie needs help finding Flippy but then Buster rushes in and says that there’s a truck going around town carrying illegal Supa Brain Dots. Dog Man barks in confusion because he didn’t know they were illegal and Chief explains that after the events of Movie 2, the mayor banned them and he tells Dog Man to get the illegal brain dots instead. Dog Man nods in understanding, salutes Chief, and then leaves. Meanwhile, Piggy uses living spray to bring Flippy back to life and he tells Flippy to join them but Flippy bursts into a rendition of the song Solo by Clean Bandit ft Demi Lovato and then finally outright says no and throws Piggy into a wall using his psychokinetic brain powers. Crunky and Bub rush to attack him but he throws them into a wall as well and leaves.
Once Piggy wakes up after passing out, he says to go to plan B and Bub presses a button and a pair of glowing red eyes turn on in the distance. At the same time, Fair Fairy is sulking by a pond and monologues about how she won’t fade into obscurity just because she was cancelled and she asks the Lord to give her a tool by which she can smite her enemies. Soon, twenty-two different tadpoles come up to the surface and ask if she’s their mommy and she starts screaming. Meanwhile, Dog Man is locked in a very intense car chase with the truck carrying Supa Brain Dots and he barks into his megaphone, telling the truck’s drivers to pull over. The driver says no and speeds even faster, knocking down some street lamps, popping water hydrants, and scaring civilians.
Dog Man growls and speeds more to catch up with the truck and they eventually make their way towards the outskirts of the city. Near a pond, Dog Man gets close enough and rams his car into the truck causing it to go off the road and crash into a tree. All the truck’s contents then spill out and the brain dots spill into the meadow where the tadpoles are interacting with an uninterested Fair Fairy. Dog Man then handcuffs the driver and the guy in the front seat, takes them into his car which has some damage but not enough to render the car undriveable, and drives off. After the Supa Brain Dots flood into the pond, the water turns purple and the tadpoles start freaking out.
Fair Fairy leaves because not only did she not want these tadpoles, now they’re contaminated and the tadpoles start crying. However, one of the tadpoles starts floating and the others are shocked and then they start floating too. Another tadpole grabs a rock with his powers and he starts laughing evilly and the others join in. They start discussing ways to get Fair Fairy back and one tadpole points to a tree and she suggests they bring it to life. She then starts bringing it to life and the others join her in bringing it to life until soon the tree fully comes to life and the tadpole who brought it to life orders it to go after Fair Fairy and capture her.
The tree salutes the tadpole and sprints after Fair Fairy, who’s ranting to Downward Dog about not falling into obscurity and killing Sam Hamilton. Downward Dog then says she has more pressing issues to worry about like a giant walking tree chasing her and Fair Fairy laughs and turns around. When she sees the tree, she screams and tries to run but the tree grabs her and brings her to the infuriated tadpoles. The tadpoles lash out at her and say that she’s going to mother them whether she likes it or not and Fair Fairy, sensing an opportunity, says she’d love to be their mother but they have to help her get revenge and the tadpoles agree. Then she says they need a name for the tree and she suggests Darkroot Sourwood but the tadpoles are like nah and decide on Barky McTreeface.
Fair Fairy begrudgingly accepts and says that they need to get going but the tadpoles say they need to come up with a jingle for him first and she screams as they start singing. Meanwhile, Petey is doing dishes while Grampa is watching a reality tv show about celebs on a yacht having drama and Lil Petey comes up to Grampa and says he made a comic for him. Grampa pauses the show to look through it and then says the drawings are crappy and look like they belong to a caveman who was frantically using a chicken to draw and Lil Petey gets sad. Grampa then says he does have a use for it though and Lil Petey’s face lights up and then falls again when Grampa says he’ll use it as a coaster for his beer can. Lil Petey then says, while choking back tears, that he made it as a gift to Grampa because he loves him.
Grampa says that if he really cared, it would’ve looked like something straight from Da Vinci but instead it looks even worse than a caveman’s and Lil Petey starts crying and runs up to his room. Grampa calls him a puny little wuss who can’t handle the truth and goes back to watching his show. Petey then finishes the dishes, calmly walks over to Grampa, and punches Grampa so harshly to the point that Grampa breaks a couple bones. Grampa gets infuriated and starts cussing and he asks Petey why he did that and Petey says because he’s a dirtbag and Grampa says he needs the doctor and Petey gives him a copy of Alexander Graham Bell’s original telephone and says to call the doctor himself and pay for it himself. Grampa tries to call on it but the call fails and Grampa says it doesn’t work and Petey says yeah, it only works 2.9% of the time so Grampa better hope he’s lucky.
Petey then goes upstairs to Lil Petey’s room where he’s crying into his pillow. Petey asks Lil Petey if he’s ok and Lil Petey asks Petey why Grampa is so mean. Petey says Grampa was always like that and he cheers Lil Petey up by reading from a book called “101 Best Poop Jokes”. The duo then spends the rest of the time laughing and telling jokes. Meanwhile, Fair Fairy and her crew go on a rampage and start destroying buildings and Flippy, who’s been hiding in the shadows, comes out and asks Fair Fairy if he can join them and she says: “the more, the merrier”.
Then, the FLEAS, who have also been rampaging through town with their robo brontosaurus, cross paths with Fair Fairy and co. and the FLEAS get mad upon seeing Flippy and Piggy yells at his crew to attack. Fair Fairy and her crew then start fighting with the FLEAS with scenes like one tadpole hoisting Piggy up into the air and Barky McTreeface throwing Crunky the robo-brontosaurus into a building. Then, the heroes come and Fair Fairy screams at everyone to come together and stop fighting so they can destroy the do-gooders and the villains agree and start working together to pummel the heroes. Dog Man tries trapping one tadpole in a net but the tadpole ends up flying and bringing Dog Man into the air while Chief aims a grappling hook at the robo-brontosaurus but the robo-brontosaurus snaps the rope and starts chasing Chief. Sarah, upon seeing this, calls Hudson, asking him to come and help them defeat Fair Fairy and the other villains.
Then, she turns to 80-HD, who is currently shooting lasers at Flippy, and tells him to go get Petey and Lil Petey. 80-HD salutes Sarah and then blasts off to go get Petey and Lil Petey, who are wrapped up in a blanket on a couch watching a movie and eating popcorn. 80-HD breaks through the window and Petey gets mad at him for not using the door. 80-HD then turns off all the lights in the house except one and turns a nearby AEKI box into a stage. Then, he grabs a bunch of paper and popsicle sticks and turns them into puppets and puts on a puppet show to tell them what’s going on.
Petey asks why he didn’t just make a comic or something but Lil Petey interrupts him and tells 80-HD to take them there. 80-HD salutes Lil Petey, grabs him and Petey, and blasts off to the fight scene. Meanwhile, Grampa is at a payphone outside of the hospital, in a full body cast, calling his lawyer and asking her to sue Petey for attempted patricide and the lawyer says he has no proof that Petey was attempting patricide. Grandpa gets mad and cusses out the lawyer before abruptly hanging up. He languishes in anger on a nearby bench until he hears a voice that sounds like Lil Petey’s and he looks up to see Petey and Lil Petey flying on 80-HD.
As soon as he hears Lil Petey’s voice, evil surges through Grampa so much that he breaks out of his full body cast, perfectly fine. Then, he follows them to their landing spot at the fight scene. Then Petey, Lil Petey, and 80-HD arrive at the fight scene where Dog Man is biting at Barky’s legs, Sarah’s judo flipping Crunky, and Chief and Hudson are shooting bazookas at the villains. Petey then turns to Sarah, who just finished judo flipping Crunky, and asks what’s going on and Sarah says some random old lady, with the help of the FLEAS, Flippy, mutant tadpoles, and a giant angry tree, is attacking the city. Petey then nods and pulls out a ray and starts going around shooting at the villains.
Meanwhile, Lil Petey is sitting at the edge, watching the action and asking himself what he can do to help. 80-HD sees this and gives Lil Petey a pair of razor-sharp claws to fight with. Lil Petey puts them on, extremely happy, and thanks 80-HD before chasing a bunch of tadpoles. He keeps chasing them until there’s only two tadpoles left and he chases them into a dead end. Lil Petey’s about to slice them but then the two tadpoles beg Lil Petey not to slice them and upon hearing their voices, he realizes their voices sound like children’s voices and he sheathes his claws. Lil Petey apologizes for almost killing them and then asks what their names are and one tadpole says she and her brother, the other tadpole, don’t have names and neither do any of their siblings.
Lil Petey says that sucks and then decides to call the female tadpole Molly and the other tadpole Nicholas and Molly says she likes it and Nicholas says he likes it too and the three of them start telling each other jokes and laughing. Meanwhile, Grampa comes up to the villains and offers to join them and Fair Fairy laughs and says she’s too powerful, Piggy says they don’t need an old geezer to help them, and Flippy tells Grampa to buzz off and uses his powers to throw him against a lamppost. A tadpole then comes up to Fair Fairy and tells her that two of the tadpoles were seen fraternising with the enemy. Fair Fairy gets enraged at this and tells the others to keep doing evil stuff because “Mama needs to give her babies a lecture”. Fair Fairy comes over to Lil Petey, Molly, and Nicholas and asks Molly and Nicholas what the heck they’re doing.
She then carries off Lil Petey and drops him into Barky McTreeface’s hand with the other heroes and she smiles before Grampa calls her over because he GateRushed some Chinese food to celebrate. Fair Fairy licks her lips and goes to join them and Molly and Nicholas use this opportunity to free Lil Petey and Zuzu and the four of them run away to Dog Man’s house where they try to come up with a plan. Lil Petey says that the only way the good guys are going to win is if the other tadpoles lose their powers or they get convinced to join the good side because he who controls the tadpoles controls the people. Molly and Nicholas say they’re siblings are hard as rocks and they’re going to be hard to convince and Lil Petey says then their siblings will have to lose their powers. He then asks the Supa ‘Puter to generate an antidote for Supa Brain Dots and the Supa ‘Puter says it’ll take a couple minutes and plays upbeat techno music from its speakers while the group waits.
Soon, the computer generates the antidote and Lil Petey takes it and asks the others what the plan is. Meanwhile, the villains are still enjoying their meal and laughing about villain stuff and Flippy remarks that the geezer (Grampa) actually did something worthwhile. Grampa gets mad and is about to respond but then Lil Petey, disguised as a hula dancer, gives the villains chocolate milkshakes inside coconut shells and then stands in front of them and says in honor of poop, he’s going to do a hula dance. Grampa says this is ridiculous but Piggy tells him to shut up and the others watch Lil Petey dance. Meanwhile, Molly and Nicholas tell their siblings they need to go back to their home pond for protection and Molly and Nicholas will sacrifice themselves to keep watch over the heroes.
The other tadpoles are initially apprehensive but then they thank Molly and Nicholas and they go to the pond where Zuzu has just finished dumping in the antidote. Meanwhile, the villains are still watching Lil Petey’s hula dance when they notice that Barky McTreeface has turned back into a normal tree and the villains start screaming as the heroes start rushing at them. Hudson shoots a bazooka that explodes the robo-brontosaurus, Chief shoots a tranquilizer disguised as a pen at Fair Fairy, Sarah Hatoff and Petey go after the FLEAS. Grampa tries to run as well but Lil Petey rips off his disguise and unsheathes his claws, pointing them threateningly at Grampa’s throat. After all the bad guys are arrested, Zuzu, Molly, and Nicholas reunite with the main cast and they cheer and take a moment to celebrate Lil Petey, Zuzu, Molly, and Nicholas. Then, the group turns to Flippy, who hasn’t run away nor been arrested yet, and they ask him why he hasn’t tried to run away.
Flippy laughs and says that they’ll need someone to control the tadpoles and he has expertise on psychokinetic brain powers. Petey, Dog Man, Chief, Sarah, and Hudson all look at each other and begrudgingly agree to let Flippy raise the tadpoles and Molly and Nicholas decide to go with Flippy to keep an eye on him. The three of them leave to go to the pond and Hudson leaves in a helicopter. Dog Man, Chief, Sarah, Petey, and Lil Petey then go to a secluded area to look at the stars and the movie ends