When the film opens, a St. Bernard puppy (later named Beethoven) and some other dogs are stolen from a pet shop by two thieves. Beethoven manages to escape from the thieves and sneaks into the home of the Newton family. The father, George, doesn't want the responsibility of owning a dog, but his wife, Alice, and children convince him. While trying to name the dog, the youngest daughter, Emily, plays a portion of Ludwig van Beethoven's Fifth Symphony and the puppy barks along, thus he is named Beethoven.
Initially, George only focuses on Beethoven's negative aspects: scratching the door, shedding on the furniture, dining off the kitchen counter, and otherwise chewing the house apart. He becomes especially frustrated with Beethoven after the dog storms in on a barbecue the Newtons are hosting for two potential investors for George's air freshener company. However, the rest of family grows more attached to Beethoven, who helps the children overcome their problems in various ways, such as frightening off the bullies that are bothering Ted, helping the eldest girl, Ryce, talk to her crush, and saving Emily's life when she falls in their babysitter's swimming pool.