A group of puppies are stolen from a pet store by two thieves. A St. Bernard (dog) puppy escapes and sneaks into the Newtons' home. The workaholic father, George Newton, doesn't want the responsibility of owning a dog, but his wife, Alice, and their children, Ryce, Ted, and Emily, convince him. They give him the name “Beethoven” when Emily plays a portion of Ludwig van Beethoven's Symphony No. 5 (Beethoven) on the piano and the dog barks along to it.
In the Newton family home, George, Alice, Ryce, Ted, Emily, and Beethoven are all well adjusted to living together. Beethoven sneaks out and meets Missy, a beautiful female St. Bernard, and her owner Brillo. Regina, Brillo's future ex-wife, arrives, takes Missy, and is seeking $50,000 in the settlement. She has retained full custody of Missy and only plans to transfer her to Brillo once the divorce is finalized.
Richard Newton, George's brother, prepares for a road trip from Denver, Colorado, to a Newton family reunion in California with his wife Beth and children Sara and Brennan. He models the entire vacation based on a memorable family road trip from his childhood in 1967, and rents a large, state-of-the-art recreational vehicle for the occasion. Before the family leaves, they receive a cage from George containing Beethoven, who Richard's family is supposed to deliver to California as Beethoven's owners are stuck in Europe. Beth and Brennan disapprove of the plan, and Beth convinces Richard to leave Beethoven at a dog kennel until George can retrieve him.
A father checks in with his two children before school. The family includes a girl, boy, artist father, and campaigning mother, and a St. Bernard (dog) dog named Beethoven (film), who they are keeping for a relative family. The children love Beethoven, but their parents do not, and want to get rid of him. So the children begin taking the dog to Obedience Training led by a former army sergeant. Brennan falls in love with a girl (Hayley), while Beethoven literally destroys the obstacle course in one day. To top it all off Beethoven hits the army sergeant in the groin with a leash causing him to kneel over in pain.
When Sara Newton and Beethoven are sent to spend the summer with Sara's uncle, clumsy mechanic Freddy Kablinski (her mom's brother) in an old mining town called Quicksilver, the mischievous Beethoven "digs up" a clue to the whereabouts of the legendary, hidden fortune of two bank robbers named Moe and Rita Selig. Now everyone wants to be his best friend as his discovery unleashes a frenzy of treasure hunting among the community's creatures. With help from Uncle Freddy and a boy named Garrett, Sara and Beethoven try to help uncover a secret that has been in the crazy little town for years.
Eddie, a struggling animal trainer and widowed father, is an assistant animal trainer to Sal DeMarco, an untalented, egocentric animal show host. Both men have been hired to train the animals for a film called ''Frizzy, The Bichon Frise'': Sal as the dog handler, and Eddie is given only the responsibility of training the lizard. When Frizzy is kidnapped, Sal quickly claims that Eddie was responsible for the dog handling. Eddie is promptly fired. The director refuses to pay the ransom and decides to instead hold auditions for the new Frizzy.
A Christmas Elf accidentally takes off in Santa's sleigh, crash lands in a small town, and loses the magic toy bag. Beethoven must rescue the Elf, recover the bag from greedy crooks, and return the sleigh to Santa in time to save Christmas
Beethoven the St. Bernard's adventures in the capital of filmmaking and in a small coastal village who lives out of a dream of a pirate treasure hidden two centuries ago.