NOTE: This spoiler was sent in by MadMumbler.

The movie opens with a para-military assault team abducting a 300 year-old sheepdog from a small, remote town in Tibet.

Dave Douglas (Tim Allen) is a driven DA who wants to go on to bigger things. He is a loving father, but waaaaay out of touch with his family. His teenage son wants to sing in the musical Grease, but is too scared to tell his father that. (His father loves football and pushes him to do well in school so he can play football.) His wife is frustrated by how he forgets appointments with teachers about the kids. He ends up trying the case of one of his daughter's favorite teachers, who is accused of breaking into a research lab and starting a fire.

The teacher claims the research company is using animals for freaky experiments. No one believes his claims, and company rep Dr. Kozak (Morton Downey, Jr.) discounts them.

The dog, of course, is in the research lab. The research company's chief is sick and looking for a cure. Kozak has a team working on figuring out why the dog is so old and healthy. In the process, they have created quite a few odd creatures, like a barking, panting, friendly cobra with a dog's tail, barking chimps, mutated rabbits, and the like. Everything starts turning into a dog.

During a protest at the lab for the teacher, Douglas spots his daughter and tells her to leave. She grabs her boyfriend, but instead of leaving they try to break in. In the meantime, the dog escapes, finds the daughter, and she takes him home.

Only problem is her father HATES dogs. The dog ends up biting Douglas, who then has him sent to the pound. What he doesn't know is that the bite transferred DNA to him that will change him too.

At a visit to the pound the next day, Douglas changes into a sheepdog identical to the one from the lab. Kozak's team finds the real dog and takes him back to the lab.

Meanwhile, Kozak is nuts, and poisons his boss so everyone thinks he has a stroke. He wants to take over the company and make a fortune on the formula for the fountain of youth.

Douglas ends up seeing his kids from a dog's point of view. He tries to tell them he's now a dog, but they don't get the message.

Douglas turns back into a human, then into a dog, and into a human. He finally figures out that "dog" stuff triggers him, and in a quest to get answers, he breaks back into the lab by having a bum throw a stick for him for "fetch." The transformation's caught on video, and when Kozak and his team view it, they know Douglas is onto them. And now Douglas knows the teacher WAS right.

Douglas finally gets through to his kids that he's a dog. In the meantime, Kozak's team dognap him. He bites Kozak at the lab. The kids go to rescue him. While at the lab, he tries to escape, manages to free the animals, but is recaptured. His kids get the other dog thinking it's their dad, and go tell their mom what happens.

Douglas, with the help of the animals, manages to escape as a human and races to the courthouse in his car with the animals. He calls his wife and kids, tells them to meet him at the courthouse with clothes and the dog. He gets caught in a traffic jam and changes into a dog to get to the courthouse in time.

In court, he baits Kozak, knowing he bit him, into reacting like a dog. Kozak tries to leave only to find out he's now got a huge shaggy tail. The teacher is released, Kozak is arrested, and the Douglas and his wife and kids take the dog to Hawaii for a short vacation before the dog returns to Tibet.