Wilby Daniels is constantly misunderstood by his father, Wilson. Wilson thinks Wilby is crazy half the time because of his elder son's often dangerous inventions. As a retired mailman who often ran afoul of canines, he has a hatred of dogs, and he can't understand why his younger son, Montgomery "Moochie" wants a dog so badly.
Wilby and his self-centered rival Buzz Miller take a new French girl, Francesca Andrassé, to the local museum. Wilby gets separated from the other two, who leave without him. Wilby ends up in a new wing, where he encounters former acquaintance Professor Plumcutt (whose newspaper Wilby used to deliver), who tells him all about mystical ancient beliefs, including the legend of the Borgia family, who used shape-shifting as a weapon against their enemies.
On the way out, Wilby collides with a table of rings, ending up with one in the cuff of his pants which he finds later. It is the cursed Borgia ring, and when he reads the inscription on it, he turns into Chiffon, Francesca's shaggy Bratislavian sheepdog. Confused about what has happened, Wilby as a dog goes to Professor Plumcutt, who says he has invoked the Borgia curse upon himself, which can only be broken through a heroic act of selflessness. After getting chased out of his own house by his father (who hasn't realized the dog is actually his older son), Wilby has a series of misadventures, as he constantly switches back and forth between human and dog forms. Only Moochie and Professor Plumcutt know his true identity when he is a dog, as Wilby has spoken to them both in dog form. Finally, he goes to a local dance (as a human) and while dancing turns into a dog. He runs out quickly, and goes home.
The next day, Wilby (as a dog) and Moochie are talking when Franceska's butler Stefano comes out and drags Wilby into the house. Stefano and Dr. Valasky (Franceska's adoptive father), discuss plans to steal a government secret, and Wilby (still a dog) overhears. Unfortunately for him, he transf...