Songwriter Roger Radcliffe lives in a bachelor flat in London, along with his dalmatian, Pongo. Bored with bachelor life, Pongo decides to find a wife for Roger and a mate for himself. While watching various female dog-human pairs out the window, he spots the perfect one, a woman named Anita and her female dalmatian, Perdita. He quickly gets Roger out of the house and drags him through the park to arrange a meeting. He and Anita fall in love and get married.
Later, Perdita gives birth to a litter of fifteen puppies. That same night, they are visited by Cruella De Vil, a wealthy former schoolmate of Anita's. She offers to buy the entire litter, but Roger says they are not for sale. A few weeks later, she hires her henchmen, Jasper and Horace, to steal them. When Scotland Yard is unable to find them, Pongo and Perdita use the "Twilight bark", a canine gossip line, to ask for help from the other dogs in London.
Colonel, an old sheepdog, along with his compatriots Captain, a gray horse, and Sergeant Tibbs, a tabby cat, find the puppies in a place called Hell Hall (Cruella's abandoned and dilapidated family estate, also known as The De Vil Place), along with many other dalmatian puppies that she had bought from various dog stores. When Tibbs learns they are going to be made into dog-skin fur coats, Colonel quickly sends word back to London. Upon receiving the message, Pongo and Perdita leave town to retrieve their puppies. Winter has come, and they have to cross the Stour River which is running fast and laden with slabs of broken ice. Meanwhile, Tibbs overhears Cruella ordering Jasper and Horace to kill the puppies that night out of fear the police will soon find them. In response, Tibbs attempts to rescue them while Jasper and Horace are preoccupied watching television, but they finish their show and come for them before he can get them out of the house. Pongo and Perdita break in and confront Jasper and Horace just as they are about to kill the puppies. While the adult dogs attack them, Colonel and Tibbs guide the puppies from the house.
After a happy reunion with their own puppies, Pongo and Perdita realize there are dozens of others with them, 99 altogether including their own. Shocked at Cruella's plans, they decide to adopt all of them, certain that Roger and Anita would never reject them. They begin making their way back to London through deep snow; all open water is frozen solid. Other animals help them along the way. Cruella, Jasper, and Horace chase them. In one town, they cover themselves with soot so they appear to be labrador retrievers, then pile inside a moving van bound for London. As it is leaving, melting snow clears off the soot and Cruella sees them. In a rage, she follows the van in her car and rams it, but Jasper and Horace, who try to cut it off from above, end up crashing into her. Both vehicles are smashed to smithereens and fall into a deep ravine; and battered, bruised and stranded, Cruella and her henchmen are defeated at last. Cruella yells in frustration as the van drives away but Jasper finally gets the courage to tell her to shut up.
Back in London, Roger and Anita are attempting to celebrate Christmas and his first big hit, a song about Cruella, but they miss their canine friends. Suddenly, barking is heard outside and, after their nanny opens the door, the house is filled with dogs. After wiping away the rest of the soot, they are delighted to realize their companions have returned home. After counting 84 extra puppies, they decide to use the money from the song to buy a large house in the country so they can keep all 101 dalmatians.