In a frame story, Branch Hawksbill, Andy Devine, assistant director of a city zoo, is visited by a father and son who found a bear cub during a camping trip. The father argues that the cub should be donated to the zoo, but Tim resists, insisting that the zoo would be like a prison. Branch then tells Tim about another boy who faced a similar decision, using the tale to illuminate the zoo’s own struggle.
Chris Carlyle [Jay North] lives in the countryside and has befriended Sunshine, an adult male mountain lion who has grown up as a pet. Because Chris has treated the cat as a companion for years, Sunshine is unusually tame and accustomed to human food. When Chris’s parents announce they’re moving to the city, Chris worries that Sunshine will not survive in the wild. In secret, he loads Sunshine onto the family’s truck and brings him to their suburban new home, where the mountain lion’s presence quickly frightens the neighbors. The city’s zoo director, Dr. Del Hartwood [Martin Milner], along with his assistant Isobel Moon [Joyce Meadows] and head zookeeper Branch Hawksbill, persuade Chris to donate Sunshine to the zoo.
At the zoo, Chris is saddened to see animals confined to cramped cages made of chain-link fencing. After a nightmare about being locked up himself, he resolves to free Sunshine. The staff, recognizing the bond between boy and cat, offer him a summer job as a junior zookeeper. Dr. Hartwood complains to the Parks and Recreation Commission that the zoo is under-funded and its facilities are outdated, which has led to injuries among the animals, but the politicians seem unlikely to help unless public opinion is stirred.
A trio of troublemaking boys harasses the animals, feeding cigars to a hippopotamus. While the staff deal with them, Chris steals Branch’s keys and opens all the cages, letting the animals roam the city. This sparks a string of comic escapades: an ostrich swallows a portable radio, a bear rides a bicycle through the streets, a zebra wanders into a family’s kitchen, an Asian elephant drinks a man’s bathwater, and several primates invade a toy store. Public panic erupts, and the police rush to round up the animals before more damage is done. Councilman Pew blames Dr. Hartwood for the zoo’s woes and calls for his resignation. After hours of chaos, most animals have either returned to the zoo or been recaptured. The police corner Chris and Sunshine in a warehouse and are ready to shoot the mountain lion, but Hartwood manages to calm Sunshine by feeding him whipped cream.
To shield Chris and Hartwood, Branch steps forward and confesses that he released the animals to draw public attention to the zoo’s plight. At Branch’s trial, Chris speaks up, unwilling to let Branch take the blame. Hartwood passionately defends Chris, praising his love of animals and noting that his actions have jolted the public’s apathy into concern for the zoo. The judge dismisses the case and orders Chris to spend two hours a day working at the zoo for the rest of the summer.
As Branch finishes the story, he reveals that the city has approved a new, improved zoo. The fences have been replaced by open, roomy exhibits that invite closer, more humane viewing. Chris works there, happily caring for Sunshine in the mountain lion’s new home. Tim, moved by the changes, agrees to donate the bear cub to the zoo, confident that it will be safe and well cared for.
