Evil scientist Dr Kozak (Robert Downey Jnr) has kidnapped a sheepdog that has reported been alive for some 300years, and performs tests on the animal to discover its secret - but his lab can only replicate other, awful mutations on other animals. The dog escapes and is rescued by the daughter of the Deputy District Attorney Dave Douglas (Tim Allen). After Dave is bitten by the animal, something strange starts to happen to him...

He convinces his kids that he has turned into a dog, with the help of some scrabble pieces. He then chases another dog from his front lawn and is zapped by the evil scientists, then taken off to the lab.
  Banged up, he transforms himself back into a human and helps the other animals escape, but is zapped with the rod again. This time the monkey comes to rescue him, and they all drive off together to the courthouse to prosecute the evil scientist.
  On the way, they almost crash in heavy traffic - so Dave dumps the car, transforms back into a dog, and races off.  He meets his family outside the courthouse, says "I love you" Scooby-Doo style to his wife, then is back as a naked Tim Allen again.
  His case doesn't go so well and he is escorted away by security. He grabs a baton and runs back into the room, hurling it away, and in the most weird scene you'll ever see, Robert Downey Jnr has a frenzied look on his face and chases after it on all fours. He then starts to turn into a dog, and the case against him is upheld.
  Outside, the monkey has somehow got the car to the front of the building in a battered condition, and is given a ticket by the cops.
  Dave takes his family to Hawaii. They watch the 300-year old dog do some surfing, and then he catches a frisbee in his mouth, proving that even old dogs can learn new tricks! (sorry, couldn't resist)