As the Daniels family returns home from a refreshing vacation, an unsettling atmosphere greets them. Their residence has suffered from not one, but two burglaries, leaving them feeling exposed and betrayed. This particularly infuriates Wilby (Fred MacMurray), who suspects that the district attorney, John Slade (Alexander Scourby), may be entangled in the criminal activities, thanks to his rumored associations with organized crime. The normally composed Wilby finds himself engulfed in a whirlwind of indignation and a craving for justice, propelling him to take a bold step and run for the position of district attorney.
Meanwhile, the unscrupulous duo behind the Daniels family's misfortunes, Freddie and Dip, stumble upon a mysterious Borgia ring during their visit to a local museum. Mistaking it for a valuable treasure, they steal the exquisite item, blissfully ignorant of its profound importance. The ring, with its curious inscription, "In canis corpore transmuto," seems like a bizarre curse; however, it presents a uniquely ominous challenge for Wilby.
As he gears up for his television debut as a candidate for district attorney, Wilby discloses to his wife Betty (Jean Hagen) the secret of his bygone shape-shifting abilities. Initially met with skepticism, Betty is utterly shocked when the inscription is read aloud, triggering Wilby's transformation into a shaggy dog right before her eyes. In a frantic rush, he escapes, with his hair cascading into a thicker mane, leaving his son Brian (Kevin Corcoran) utterly bewildered.
Transitioning back into his human form, Wilby is left grappling with the unsettling reality that he could morph into a canine at any moment. Determined to regain control over his life and reputation, he embarks on a quest to retrieve the errant ring while maneuvering through a hazardous landscape filled with political complexities and citizen scrutiny. With his family, career, and fundamental identity at stake, Wilby must wrestle with the beast hidden within and restore balance to his existence.
As Katrinka locks her gaze onto the ring, her life descends into chaos, paralleling Wilby's own strange encounter as he finds himself addressing a ladies gardening club in his human guise. Just as he begins to transform into Elwood once again, chaos ensues, sending him into a hasty retreat. At the same time, Tim (Tim Considine), blissfully unaware of the uproar, momentarily wanders off, only to confront a mystified Elwood, deepening the confusion.
Additionally, Raymond Slade (James Westerfield), Wilby's agent, grows increasingly wary of his client's unusual disappearances, sensing a troubling pattern. The pursuit of the ring becomes ever more frantic, leading Katrinka and her associates on a wild mission, culminating in a pie-filled ruckus at a fundraiser for Slade. Amid the uproar, the miscreants manage to recover the ring and scheme to hand it off to an undercover detective.
Once again, the museum takes possession of the ring, its inscription spoken aloud, sparking intrigue. At the police station, Wilby's presence coincides with his unexpected transformation back into Elwood. This time, Raymond deduces the peculiar connection after overhearing the museum curator explain the curse's origins, telling the tale of a young man who transformed into a sheepdog long ago.
Raymond invites Wilby to his office, encouraging him to bow out of the campaign. However, Wilby stands firm, intent on investigating Slade's web of crime once he assuming office. Slade, eager to see Wilby's transformation, reads the inscription aloud, which leads to a tumultuous turn of events, as he calls the local dog pound, aiming to trap Wilby in his canine form indefinitely.
With Slade's machinations closing in on him, Wilby finds himself ensnared between the district attorney's grasp and animal control. Eventually, he is captured and taken to the pound, where he surprises himself by finding comrades among the other dogs. United in their quest for liberty, they band together, rallying to aid Wilby in his escape.
With the support of his loyal companions Brian and Tim, who still hold the belief that Elwood can talk, Wilby delves into the truth about Slade's ties to organized crime. Employing clever tactics, he orchestrates a sting operation, leading Slade into Roshak’s warehouse under the pretense of a legitimate meeting. As Slade unveils his true intentions, Wilby capitalizes on the opportunity, capturing incriminating evidence on film to hold this criminal mastermind accountable.
Meanwhile, the canine brigade from the local pound becomes an essential part of Wilby's team. Slade, having recited the magical inscription too many times, comically transforms into a bulldog, underscoring the notion that even the highest-ranking individuals can succumb to the whims of fate.
As the surreal adventure draws to a conclusion, Wilby's relentless efforts culminate in an electoral victory, securing him the title of district attorney. In a fitting twist, Slade finds himself apprehended by law enforcement for speeding—a poetic closure to his criminal endeavors. As Tim celebrates his own engagement to the enchanting Katrinka, Wilby forms a new family with his canine allies, forever transformed by their shared journeys and unyielding bond.
