Lloyd Christmas is a limo driver in Providence who becomes infatuated with his passenger, Mary Swanson, as he drives her to the airport. Mary is heading home to her family in Aspen, after dropping off a briefcase with a large sum of cash at the airport terminal, as ransom money for her kidnapped husband. Lloyd witnesses the drop-off, and, thinking Mary had lost the briefcase by mistake, intercepts the package before the kidnappers pick up their pay, dashing ahead of them to snag the briefcase. Lloyd is unable to catch Mary in time, and is left on the runway of the airport with briefcase in hand (after falling out of the terminal, having run out of an empty jet bridge).

Harry Dunne, Lloyd's roommate, is in the pet grooming business, and has recently spent his life savings converting his van (a 1984 Ford Econoline) into a sheepdog. Both Lloyd and Harry quickly lose their jobs due to preventable accidents, and the two are distraught over their situation. Thinking Lloyd is a "professional" hired by the Swansons, or perhaps an FBI agent, the kidnappers exact revenge on Harry and Lloyd by beheading Harry's pet parakeet while the pair are out looking for new jobs. Lloyd convinces Harry they should leave their messed up lives in Providence behind and head for Aspen to return the briefcase to Mary, unaware the locked briefcase contains enough money to support them both.

On their way to Aspen, Harry and Lloyd have several misadventures, inadvertently kill one of the kidnappers by accidentally feeding him rat poison, and wind up separating when Lloyd takes a wrong turn and drives them to Nebraska instead of Colorado. Out of gas, Harry begins walking home, but Lloyd is able to trade the van for a small scooter, and the two drive to Aspen.

Unable to remember Mary's last name, or locate her in the phone directory, the two spend a cold night in a park. They end up in a tussel, and in the process knock open the briefcase and discover the money. The two decide to "spend it responsibly" and keep track of all expenditures with "I.O.U's", but they end up living extravagantly, they check-in at a luxury hotel, buy a Lamborghini, have full body make-overs, and get expensive, off-color suits. Lloyd tries to use the money to woo Mary before revealing he has the briefcase, but Mary ends up falling for Harry, whose stupidity she mistakes for intentional humor.

Eventually the two are found out by the kidnappers, who hold the three hostages in their hotel room at gunpoint. When Lloyd reveals all the money is gone, replaced with I.O.U's written on dirty napkins, the kidnapper freaks and almost kills the pair. FBI intercept, and Mary is reunited with her husband, much to Lloyd's dismay, who didn't even realize Mary was married.

With no money, the two begin to walk home. Along the way, they inadvertently turn down a chance to be oil boys for Hawaiian Tropic bikini models, instead walking off together, thinking to themselves how two lucky guys will get to tour all over the country rubbing down the girls before shows. Lloyd reassures Harry that "someday we'll get our big break too, we've just got to keep our eyes open".

• Lloyd Christmas is an illiterate and somewhat mischievous man who has apparently been fired from several jobs due to his lack of intelligence and his unwillingness to work "40 hours a week", the most recent of which is working for a limo company. He melodramatically falls in love with Mary while driving her to the airport, and becomes convinced he is destined to track her down, return her misplaced briefcase, and spend his future with her. He and Harry are the main characters.
• Harry Dunne is a good-natured dog groomer, and best friend to Lloyd. What he lacks in common sense, he makes up for by being a superior linguist to Lloyd. He, along with Lloyd, plans to open up his own pet store to specialize in selling worm farms; the store is tentatively named 'I Got Worms'.
• Mary Swanson is an attractive lady whose husband, Bobby, has been kidnapped by a family friend. She is the object of Lloyd's longing.
• Nicholas Andre is the main antagonist of the movie. He is the kidnapper of Mary Swanson's husband, as well as a long-time confidante of the Swanson family. His plot is foiled by Harry and Lloyd after they spend the ransom money and draw the attention of the FBI to the situation.
• Joe 'Mental' Mentalino, also known as 'Gas-Man', is a tall, cold-hearted criminal who works as a henchman for Nicholas Andre, the kidnapper. He suffers from severe digestive problems, including ulcers and gas. He attempts to kill Lloyd and Harry, as well as retrieve the briefcase, but is accidentally killed for his troubles when the rat poison tablets (with which he intended to poison Lloyd and Harry) are mistaken for his pills. Although he too is a man of below-average intelligence, he is still angered by the idiocy of Lloyd and Harry.
• J.P. Shay is the female accomplice of Mental. She appears as Andre's date at the wildlife benefit.
• Beth Jordan is an FBI agent masquerading as a somewhat simple-minded and overly talkative young woman who is moving to Aspen to get away from her dumb boyfriend. She befriends Lloyd in a bar as he expects to meet with Mary, having previously met Harry as they both filled up at a gas station.
• Sea Bass is presumably a trucker. He spits on Harry's hamburger after Harry accidentally hit him with a salt shaker at a roadside diner, and is later accidentally foiled by Harry again, when he attempts to sexually molest Lloyd in a truck stop bathroom. Sea Bass was portrayed by hockey player Cam Neely, who also acted in one of Jim Carrey's later films, Me, Myself and Irene, again playing the character Sea Bass.

The film was very successful at the box office, grossing $127,175,374 in the United States, and $246,400,000 worldwide, and topping the holiday season film gross.

The movie gained a 60% "fresh" rating at Rottentomatoes. While Roger Ebert gave the film only two of four stars (despite lauding Carrey's performance and the dead parakeet joke), most reviews were positive. Stephen Holden of the New York Times called Jim Carrey "the new Jerry Lewis", and Peter Stack of the San Francisco Chronicle called it "riotous", "rib-splitting", and gave the film praise for being both a crude and slapstick comedy and a "smart comedy" at the same time.

Although the film did not come away with any major American motion picture awards, it was very successful at the MTV Movie Awards. Jim Carrey won for Best Comic Performance, Carrey and Lauren Holly (a couple who would later endure a short-lived marriage) won for Best Kiss, and Jim Carrey & Jeff Daniels were nominated for Best On-Screen Duo.

In 2000, readers of Total Film magazine voted Dumb and Dumber the 15th greatest comedy film of all time.

Scenes taking place in Aspen were actually filmed in Breckenridge, Colorado and Park City, Utah. The Stanley Hotel in Estes Park, Colorado (the main influence for The Overlook Hotel in Stephen King's The Shining) was transformed into the "Danbury Hotel" for the filming of the movie. The scenes filmed in the snow were shot at Copper Mountain Resort.

Most of the external street scenes were filmed in Salt Lake City.[citation needed] The opening scene (the limo scene) was filmed on 500 East, between 500 South and 600 South. The external courtyard of Harry and Lloyd's apartment (where they give the little blind boy Petey the parrot) was filmed on the corner of that same street.[clarify] The scene with the sweet ol' lady on the motorized cart was filmed in front of Thomson & Burrows Antique Store on 270 East and 300 South. The interior shots of Harry and Lloyd's apartment were filmed in the historic Union Pacific Railway Station in downtown Salt Lake. Mary Swanson's mansion in the beginning was filmed at LaCaille Restaurant near Big and Little Cottonwood Canyon. The airport scenes were all shot at the Salt Lake City International Airport.

After the guys pull the bill-paying stunt on Sea Bass, Harry asks Lloyd where he got that idea. Lloyd tells him that he saw it in a movie. This is a reference to the movie Something Wild in which Jeff Daniels does the same thing to Ray Liotta. Harry and Lloyd are presumably named after silent comedy film star Harold Lloyd.

While parking in the airport you can over hear "The white zone is for loading and unloading passengers only, no parking in the white zone". This is a reference to the movie Airplane!.

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