Dave Douglas (Tim Allen) is a deputy district attorney in Los Angeles County who is prosecuting social studies teacher and activist Justin Forrester (Joshua Leonard) for firebombing the pharmaceutical corporation Grant and Strictland. Forrester denies this, but claims that Grant and Strictland have been engaging in illegal animal experimentation. This distances Douglas from his daughter Carly (Zena Grey), one of Forrester's students. As Dave is also a workaholic, his relationship with his wife, Rebecca, and son, Josh, are also strained.
The greedy geneticists working for Mr. Lance Strictland (Philip Baker Hall), led by Dr. Kozak (Robert Downey, Jr.), have stolen a 300-year-old sacred dog named Khyi Yang Po (a Bearded Collie) from a Tibetan monastery. The scientists determine that Khyi Yang Po's genetic sequence, when isolated and put into a vaccination, invades and alters the cells and DNA of a victim. They continue their experiments in hopes of uncovering the genetic component that allows Khyi Yang Po such a long life, potentially immortality. Khyi Yang Po escapes the lab and is picked up by Carly, who was protesting with her friends and classmates outside Grant and Strictland.
Carly brings Khyi Yang Po home. When Dave returns from work, he takes Khyi Yang Po out in the garage, then the dog runs to get the newspaper. Khyi Yang Po gives him his newspaper, and Dave reaches for it, only to be bitten on the hand by Khyi Yang Po. Khyi Yang Po's saliva infects Dave. Over the next few days, Dave realizes that he uncontrollably transforms into a sheep dog exactly like Khyi Yang Po when prompted by distractions and activities typical of a dog, such as a stick thrown through the air. These transformations can be reversed by sleeping, but Dave's human absence from his family further strains his relationships. During this time, his family, unaware of Dave's condition, continue housing Dave as a dog, thinking it's Khyi Yang Po. Dave slowly learns just how little he understands his kids and wife and resolves to make amends.
In the ongoing trial, Forrester testifies seeing the animals behaving like dogs and the presence of a bearded collie he identifies as a giant sheepdog, which sparks Dave's suspicions about Grant and Strictland. When his dog-like behavior annoys the judge, Dave is removed from the case. Seeking answers to his transformation, Dave heads to Grant and Strictland. He has a janitor help him transform so that he may sneak in through a vent. Hidden in the laboratory, Dave witnesses Dr. Kozak injecting company president Lance Strictland with a drug that will paralyze him for several months, giving Kozak enough time to usurp control of the company. After viewing security cameras, Kozak and his minions realize that Dave is capable of transforming into a copy of Khyi Yang Po.
When Dave comes home, still in his canine form, he overhears a conversation between Carly and Josh that he and Rebecca may be splitting up. After hearing this, Dave then knocks over a Scrabble game and uses the letters to reveal his identity to his kids. But when Dave gets out of the house, he is captured by Kozak's minions. His kids attempt to save him, but end up picking up the real Khyi Yang Po instead and rush to their mom at work to report recent events.
Dave is taken to the lab to be euthanized, but Kozak has a court summons, and must deal with Dave later. Right before he exits, Kozak mocks Dave in canine form and Dave bites him in response. The other mutated animals in the lab tell Dave to meditate to reverse his transformation. Dave succeeds in returning to human form, and is able to escape with all of the other animals. He drives to the courthouse and calls his wife to ready a change of clothes for him at the courthouse, but is forced to abandon the car with the animals when they get stuck in traffic. Dave runs on all fours to trigger his transformation to get to the courthouse in time. At the courthouse entrance, his attempts to tell his wife that he loves her allows him to transform back.
In the courtroom, Dave calls Kozak back to the stand and tries to get him to admit what he has done, but Kozak manages to hide his secret. Thinking quickly, he mocks Kozak by implying he was working under Strictland's shadow, and angers him. The two begin growling at each another, and the judge, exasperated by Dave's canine behavior, orders the bailiff to remove him. Dave grabs the baliff's baton and tells Kozak to fetch, triggering a partial transformation in him and thus implicating him of illegal and unethical experimentation. The pharmaceutical company is returned to Strictland, the mutant animals enter protective custody, and Dave finally makes amends with his family.
The movie ends with the family vacationing in Hawaii with Khyi Yang Po. Josh tells his dad to fetch a frisbee he just tossed, and Dave catches it with his mouth.
The film was released in the United States on March 10, 2006. To tie-in with the theatrical release of the 2006 remake, the original 1959 movie was re-issued in the United States as a special DVD labeled "The Wild & Woolly Edition," which featured the movie in two forms: one in the original black and white, the other a colorized version. The colorized version however is not restored and suffers from age. In the UK, however, the 1959 movie has only ever been made available on DVD in black and white.
The Shaggy Dog grossed $61.1 million in the United States and Canada and $26 million in other territories for a worldwide total of $87.1 million, against its budget of $60 million.
In its opening weekend the film made $16.3 million, finishing second at the box office behind Failure to Launch ($24.4 million).
On Rotten Tomatoes the film has an approval rating of 26% based on 102 reviews, with an average rating of 4.4/10. The site's critical consensus reads, "This Disney retread has neither inspiration nor originality, but may please moviegoers under the age of ten." On Metacritic, the film has a score of 43 out of 100, based on 25 critics, indicating "mixed or average reviews".
BBC called Allen uninteresting and said he "only stops short of leg-humping in his attempts to win our affections". The film was on Richard Roeper's Worst films of 2006 list. Roger Ebert gave it 2 stars out of 4. The film earned three Razzie Award nominations including Worst Actor for Tim Allen, Worst Remake or Ripoff and Worst Excuse for Family Entertainment, but failed to "win" any of those categories. Variety Chief Film Critic Justin Chang noted: "its occasional lump-in-the-throat moments are almost effortlessly achieved, thanks to strong work from [Kristin] Davis and Spencer Breslin in particular."
The soundtrack to The Shaggy Dog was released on March 14, 2006. The entire score is by Alan Menken.
- "Big Dog" - 3:38
- "Man's Best Friend" - 3:06
- "Atomic Dog" - 4:43
- "Every Dog Has It's Day" - 2:52
- "Somethin' About You" - 3:18
- "Woof! There It Is" - 3:02
- "It's A Dog" - 2:40
- "Tibet" - 2:33
- "First Signs" - 3:00
- "Transformation" - 4:04
- "Magic Lab" - 2:19
- "Breaking Through" - 2:50
- "Kozak Gets A Tail" - 2:34
- "Meditation" - 1:06
- "Escaping the Lab" - 4:42
- "To The Rescue" - 4:54
- "Family Time" - 1:20