Wilson Daniels, a mailman who despises dogs, lives in the suburbs with his wife Frieda and sons Wilby, a teenager, and young Moochie. Wilby, whose curiosity and intelligence are matched only by his clumsiness, is constantly infuriating his father, who explodes in anger after Wilby blows a hole through the roof while testing his new invention, a missile interceptor. As Wilby patches the roof, he spots his friend, Buzz Miller, picking up the neighbor on whom Wilby has a crush, Allison D’Allessio. Suddenly realizing that the money Buzz borrowed earlier will be spent on his date with Allison, Wilby demands the money back, but the boys are distracted from their argument by the arrival of new neighbor Franceska Andrassy. While Frieda gossips to Wilson about Franceska’s father, Dr. Mikhail Andrassy, the new assistant curator of the county museum, Wilson frets over the sight of the Andrassy pet, a huge, shaggy sheepdog named Chiffon. Outside, the dog runs straight to Wilby, and when he and Buzz insist on returning Chiffon to Franceska, Allison stalks into her house in disgust. At the Andrassys’, sophisticated Franceska shows the boys her family's art collection, including a portrait of the Borgia family that features a dog identical to Chiffon. The boys drive Franceska to the museum, where Buzz steals Franceska away, leaving Wilby alone among the ancient artifacts. Prof. Plumcott is working at the museum and, upon seeing Wilby examining another Borgia portrait, explains that the Italian family was notorious for their dabbling in the occult practices of spell-casting and shape-shifting, or changing from human to animal form. Apprehensive, Wilby rushes to leave, accidentally overturning a tray of Borgia jewels. At home Wilby discovers that a ring from the Borgia jewel collection has fallen into his pant cuff, and upon reading the inscription, unwittingly sets into motion a shape-shifting spell. He immediately transforms into Chiffon, but retains his own mind and voice. Realizing that he must get help, Wilby, still in the embodiment of a dog, slips out of the house and visits Plumcott. Although the professor cannot break the spell, he guesses that an act of heroism might prove successful, as it did in the tale The Hound of Florence , and warns Wilby that he will probably change back and forth from dog to boy without warning. On the way home, Wilby frightens police sergeant Hanson by talking to him, then returns to his bedroom to sleep. Upon awakening, he is aghast to see Chiffon staring back at him in the mirror, but Moochie is thrilled finally to have a pet, and promises to walk and groom Wilby faithfully. Worried that Wilson will shoot his new "pet," Moochie hides Wilby, but Wilson soon breaks out in hives and deduces the presence of a dog. As Wilson reaches for his gun, Wilby races to Franceska’s, where he is disheartened by his breakfast of kibble. Stefano, the butler, locks “Chiffon” in a room under the stairs, and after napping and awakening as a boy, Wilby is able to escape. That night, Buzz visits to confess that he has asked both Allison and Franceska to the upcoming country club dance, and asks Wilby to accompany them so Buzz can pretend to each that the other is dating Wilby. At the dance, Buzz tells both girls that Wilby is in love with the other but is too shy to dance, thus convincing them to vie for Wilby’s attention. The other girls, noting Wilby’s sudden popularity, clamor to dance with him, and he is having the best night of his life when suddenly he takes on his dog form. Chased by both Franceska and Wilson, Wilby runs away, and when he later calls his father from a public phone, Sgt. Hanson again sees him and questions his sanity. Later, at Franceska’s house, Wilby’s appearance as a dog allows him to listen as Buzz lies to Franceska that Wilby is a troublemaker. He attacks Buzz, prompting the boy to leave, after which Wilby overhears visiting Mr. Thurm discuss with Dr. Andrassy their secret plans to smuggle a “Section 32” hydrogen missile out of the country. Wilby escapes from the house through the laundry chute, and hurriedly conveys to Moochie the information that Andrassy is a spy. Moochie tells Wilson, but his father assumes the boy is just playing, until Wilby, still a dog, approaches his father and convinces him of his plight. Wilson brings Moochie to the police department to inform them, and is proclaimed mentally unstable until he mentions Section 32, a piece of classified information. Now suspecting Wilson may be a spy, the police interrogate him until Moochie saves him by claiming that his father is a master “storyteller.” Meanwhile, Wilby returns to Franceska’s, where he hears Andrassy tell Stefano to take the missile to the museum. Just then, Wilby returns to human form and is caught by the spies, who tie him up in the dressing room and, realizing they are now compromised, grab Franceska and rush to board a secret ship heading out of the country. As soon as they leave, Moochie, who has been hiding outside, sneaks in and rescues Wilby. Despite his return to dog form, Wilby jumps into Buzz’s jalopy and chases the spies. Moochie and Buzz convince Wilson to follow, and the convoy is soon tailed by Hanson. After Hanson reports that he is chasing a driving dog, however, his chief orders officer Mercer to pursue the sergeant, and Mercer slows the policeman’s progress. Wilby arrives at the dock just as the spies are casting off, and manages to jump on board. When Franceska tries to get away, she is knocked overboard, and Wilby leaps into the sea to save her. He pulls her ashore just as Wilson and the boys arrive, followed closely by the police, who chase after the boat. Upon seeing Buzz awaken Franceska and pretend that he rescued her, Wilby attacks him, but as they fight his act of heroism returns him permanently to human form. Franceska, meanwhile, recalls her dog saving her, and finding Chiffon nearby, hugs him gratefully. Soon, Wilson and Chiffon are nationally celebrated as spy-catching heroes, and Wilson’s allergies magically disappear as he agrees to adopt Chiffon after Franceska is sent away to school. Buzz and Wilby resume their squabbling over Allison, and are dismayed to see her new boyfriend, a college man, pick her up for a date.