It starts with Chief getting his couch and while he's signing the papers, Greg and Officer Knight see the couch and they think it's for them and so they start lounging on it. Then, Chief comes in and gets mad and sends them away. Meanwhile, Evil Mayor, Dr Scum, and Petey meet up to discuss their plans for taking over the city. Dr Scum and Petey say that they are ready and Evil Mayor laughs and says that the only thing standing in their way now is Chief and Officer Knight. Petey says he'll take care of Officer Knight and he leaves and goes to a park where Officer Knight and Greg are sulking.
He plants a bomb and then runs to Knight and Greg's park bench disguised as a British nanny and tells them about the bomb. Officer Knight and Greg run to diffuse it but we all know what happens. At the hospital, the surgery that created Dog Man takes place and then Dog Man epically exits the hospital like he did in the original Dog Man movie. Dr Scum, Evil Mayor, and Petey see it on the news and start screaming and get mad. Evil Mayor throws a tantrum and tells Petey to take care of it so Petey sends his robot vacuum after Dog Man but Dog Man goes to the sea and as we know, the vacuum blows up from all the water and Petey is arrested.
Then, Evil Mayor decides to undermine Chief by secretly sending Petey invisibility spray and after Petey escapes, Chief is fired and replaced with Robo-Chief. Evil Mayor then builds all her evil stores and grants amnesty to all criminals who start going to the evil shops and committing a bunch of crimes making Ohkay City even worse than Gotham City. Dog Man whimpers and begs Robo-Chief to let him stop the chaos but Robo-Chief threatens to incinerate him if he tries anything but Dog Man keeps whimpering so Robo-Chief ties him up, leaves him in an abandoned building, and then sets it on fire. Meanwhile, Petey then unleashes some hot dog robots he brought to life and they start attacking the city and they steal some living spray and bring Philly the cheese steak gyro to life. The weenies and Philly add to the chaos but mostly just attack fast food restaurants.
Meanwhile, Sarah, an Aussie news reporter who knows martial arts and is an ex-spy, has been closely watching Dog Man and she runs to the police station to see if he’s there and when a Karen cop (Maude) tells her that “the stupid mongrel ain’t here”, she finds Chief and asks for his help to find Dog Man. Chief agrees and the two of them go into a helicopter to find Dog Man (and flirt with each other along the way) and they hear howls from a burning building and dive down. They put on gas masks (so they don’t breathe in the smoke) and rescue Dog Man. Dog Man, Sarah, and Chief then go to the center of the city to defeat the gyro and the hot dogs. Petey then watches them from his hideout and tries to assassinate Dog Man before he tries anything but Sarah uses her spy skills to stop it and gets in a fist fight with Petey. Dog Man meanwhile runs up Philly and starts biting his eyes.
Chief, however, doesn’t know how to stop the weenies until he has a flashback of watching Greg steal Officer Knight’s hot dog and he runs to the dog pound to let all the dogs out but Evil Mayor, with Robo-Chief behind her, asks what he thinks he’s doing. Chief says he needs Evil Mayor to move out of the way so he can stop the hot dogs and Evil Mayor says no and orders Robo-Chief to attack him as she runs away to Petey’s hideout and she thinks she’s safe but then she sees Sarah standing over a passed out Petey and Sarah turns around and lunges at Evil Mayor. Chief and Robo-Chief fight before Robo-Chief surrounds Chief with a ring of fire but Chief simply grabs his water gadget (a water bottle that can grow really small and fit in your pocket) and puts it out. Then, he gets an idea and throws it onto Robo-Chief, causing him to short-circuit. Then, Chief frees all the dogs from the pound who start eating the weenies.
Meanwhile, Dog Man realizes that biting Philly’s eyes aren’t doing anything so he starts trying to think of a better idea. He then sees how there’s fires everywhere and he sees all the dogs chasing after a weenie holding living spray. He gets an idea and joins the dogs in chasing the weenie with the living spray. He convinces the other dogs to let him get the leader weenie and they stop chasing the leader weenie. He eventually reaches the leader weenie, eats the head, and then takes the living spray.
He then runs back and taunts Philly into chasing him into the fire and he throws the living spray in it and run away as fast as he can before it explodes and kills Philly. He then reunites with Sarah and Chief and they all share a hug before Sarah calls the police and kisses Chief on the cheek. The cops then come and arrest Petey and Evil Mayor. Flash forward a few days and the city is all fixed up and there’s a parade to celebrate Dog Man, Chief, and Sarah. After the parade, Dog Man, Chief, and Sarah all sit on a swing under a tree on a hill and watch the sunset.
Chief is happily reading a newspaper detailing their heroism and then throws it behind the swing. They all happily look at the sunset and Chief and Sarah make some loving remarks to each other and Dog Man barks and then licks everyone. They all laugh. Then the wind picks up and the swing starts swinging and the trio giggles some more. Meanwhile, the newspaper flies away and the back of the newspaper is shown which is an article on top 10 evilest animals and on the top of the list is a certain purple butterfly fish with an evil smile. The End