In their suburban California home, George and Alice Newton frantically prepare their three children for school as the family St. Bernard dog, Beethoven, scours the house for food. While George, an air freshener entrepreneur, rehearses his pitch for a crucial business meeting, eldest daughter Ryce accepts a ride to school from her crush, Taylor Devereaux. Once the remaining Newtons leave for the day, Beethoven sneaks out the dog door and wanders the local park, where he encounters a female St. Bernard named Missy. As her doting owner, Brillo, treats the two dogs to ice cream cones, Brillo’s malicious former wife, Regina, takes possession of Missy until he agrees to her hefty alimony demands. Tearing off Missy’s pink bow, Regina returns to her luxurious condominium and locks Missy on the balcony. Beethoven follows and helps Missy jump over the ledge to accompany him on a “date,” during which they explore the town and watch a film at the drive-in movie theater. Over the next few months, young Ted and Emily Newton realize that Beethoven has been sneaking out of the house, and follow him into the basement storage room of Regina’s building, where the dogs are cuddled with four newborn puppies. Upstairs, Regina berates her dim-witted boyfriend, Floyd, for Missy’s recent disappearance until the janitor informs them of the dog’s location. Although Regina initially plans to kill the offspring, the janitor points out their purebred pedigree and suggests she sell them instead. Afraid for the puppies’ lives, Ted and Emily pack them into a covered box and sneak past Regina, who later discovers their deception and vows to get the puppies back. Back at home, Emily distracts her father while Ted builds a dog pen in the basement and shares the secret with Ryce. When the puppies refuse to drink milk, the veterinarian instructs the Newton siblings to feed them a nutritional substitute multiple times a day, which requires them to sneak out of school. Despite sharing responsibility, the effort of hiding the puppies exhausts the children, and Ryce is eventually forced to reveal them to her mother when Alice learns she has been skipping class. She agrees to keep them secret from the increasingly stressed and overworked George, who later informs the family that a friend has offered them free use of his lakeside cabin in the mountains for the upcoming Fourth of July holiday. However, his announcement is interrupted by yelping noises from the basement, and, upon discovering the four additional St. Bernards, he angrily insists they cannot afford to keep them. With persuasion from Ryce, he reluctantly agrees to keep them until they have matured, and the children assign them names: Dolly, Tchaikovsky, Chubby, and Mo. Over the next few weeks, the dogs test George’s patience by making a mess of the house and getting into life-threatening situations. Come summer, the Newtons embark on their vacation at the cabin. One night, George tells Ryce about his and Alice’s long-distance relationship during college, prompting Beethoven to lament his separation from Missy, unaware that she, Regina, and Floyd are vacationing in the same small town. Eager to find Taylor, Ryce is disappointed to see him frolicking with other girls in the lake, and quickly develops a crush on the punkish dock boy named Seth. While she walks Beethoven one night, Taylor invites her to a noisy house party, where he locks them inside the upstairs bedroom and attempts to seduce her. Beethoven, tied to a support beam, breaks free from his leash and causes the backside of the house to collapse, sending Taylor and many of the other guests flying into the lake. At the county fair, Ted uses Beethoven to save a girl from a bully, and enters George and the dog in a hamburger-eating contest. Motivated by taunts from the crowd, they win the competition, but afterward, Beethoven strays from the family to help Missy escape from Regina’s car. Spotting the puppies across the fairground, Regina and Floyd seize them from Ted and Emily and instruct them to find their missing parents in the forest, as the Newtons follow behind. When Floyd threatens to drop one of the puppies in the river, Beethoven knocks him and Regina over the cliff and into a pool of mud below, which flushes them down the rapids. Five months later, Ryce goes on a date with Seth, and Brillo visits to announce that the divorce judge has dismissed Regina’s claim and granted him full custody of Missy. Joined by Beethoven, the now nearly-grown puppies cause the house to quake as they clamber down the stairs to see their mother.