Flop vaudeville performer Len Carmody is adopted by Kelly, a trained dog abandoned by Malo's dog act. When Kelly joins Len's ho-hum song and dance, a popular comedy act is born. On the train, Len meets young Mina Van Runkel, daughter of a movie producer in desperate need of new talent (it's 1930). Successful in making "dog pictures," can Len stand being second banana to Kelly? Complications: a pallid romantic triangle...and the reappearance of Kelly's original owner.

In Jersey City, New Jersey in 1930, Len Carmody, one of the last of the vaudeville song-and-dance men, struggles to earn a living. Len, who falsely believes himself to be a great entertainer, fires his manager, Dave Gans, after Dave announces that Len has once again been fired for being too demanding. Later, Len stops fellow entertainer Milo from abusing his performing German shepherd, Kelly, and an adoring Kelly follows Len home. Failing in his attempts to rebuff the dog, Len finally takes Kelly in. The next day, Kelly breaks loose from his leash during Len's act and joins him onstage, interrupting his pompous singing with expertly timed howls and tricks. With the dog as a comic foil, Len's act proves a huge success, and he is hired by visiting Utah theater owner Joe Webb. On the train to Utah, the conductor spots the dog in the passenger section and insists Len take him to the baggage car. While the two practice their act, Kelly sniffs out Mina Van Runkel, a young woman hiding in the car to whom the dog takes an immediate liking. Len also finds Mina appealing until she explains that she has run away from school to join her father in Los Angeles, and convinced that she is a mere child, Len tries to return to his cabin. Kelly, however, barks whenever Len moves away, so Len stays with the dog and offers the cabin to a grateful Mina. Unknown to Len, Mina's father, Walter Van Runkel, runs a film studio in Hollywood, and when Mina later shows up there and insists on working instead of returning to school, he happily concedes. The studio has just switched from silent to sound film production, and the high-pitched voice of their biggest male star is proving a disaster. Desperate for a new star, Walter is swayed by Mina's passionate praise of Kelly, and instructs her to bring the dog¿and his owner¿to Los Angeles for a screen test. Len is thrilled with his luck and rushes to the studio, never guessing that Walter is more interested in Kelly than him. Soon, Kelly, a natural and gifted film performer, is beloved by the whole studio, but will respond only to Len. Forced to include Len in Kelly's first movie, Walter offers him a role opposite Walter's new wife, budding star Lucy Castle. On the set, Kelly performs perfectly but Len creates problems by insisting on creative control. Director Stu Baker eventually refuses to work with Len, but Walter convinces him to agree to Len's demands but secretly shoot the film as scripted. Although Mina warns her father that the plan will end in disaster, Walter remains unconcerned, especially after test audiences adore the finished film. Before the press conference, however, Len sees the film and, furious at being tricked, prepares to leave town. Walter arrives at his apartment and urges him to build his career around Kelly. Realizing Walter is desperate, Len demands half ownership in the studio, and then shows off during the press conference. Mina has fallen in love with Len but now, disappointed in him, decides to return to school. In the next months, Kelly delivers a string of hit films and Len receives bags of fan mail that cause him to grow more egotistical and unmanageable. As Len's fame and fortune grows, he spends less time at home, leaving Kelly forlorn. Meanwhile, Lucy, who approves of Len's extravagance and ambition, embarks on an affair with him. When Len announces a planned opera for him and Lucy, Walter explodes in anger, revealing that the fan mail has all been spurious, created by his team to keep Len happy. Finally realizing that Kelly is the real star, Len seeks solace in the local bar, complaining about Kelly while the faithful dog waits for him outside. By morning, Len is contrite, and visits Walter with the news that he will allow Kelly to work in the films by himself. He is shocked, however, by the arrival of Milo, who presents papers proving that he is Kelly's legal owner. Walter throws out Len, but Kelly refuses to work without him. Later, after Milo whips Kelly on the set, the dog runs away, prompting Walter to fire Milo and call in Mina to help. She urges him to accompany her to San Francisco, where Len is now playing in dinner theater. There, Len refuses to talk to them, but secretly searches for Kelly each night. Days later, the train station employees spot a weary Kelly, who has tracked Len all the way up the coast. Meanwhile, Mina and Walter return to Len's theater, where they overhear him urging the police to help him look for Kelly. Overjoyed that he is finally living up to her expectations, Mina falls into Len's arms. Just then, they learn that Kelly is hiding among the trains, and Len announces to the waiting audience that he must leave to find his dog. Kelly, however, has been brought to the theater by the police, and the exhausted dog rushes onstage to join Len. When they finish their number, Len lets Kelly take a bow alone.