Professor Ned Brainard's discovery of Flubber has not brought him or his college the wealth he hoped for. The Pentagon designates the invention as "top secret" and the IRS lands him with a crushing tax bill, all while no money has actually been earned from the invention. Ned clings to a possible breakthrough in Flubbergas, described as the "son of Flubber," a substance that could even influence the weather.
His experiments with the ray projector make it rain in his kitchen and garage, but the catch is clear: Flubbergas only affects makeshift clouds. When it encounters real clouds, the rain turns into a destructive shower that shatters glass all over town. This mishap draws the ire of Alonzo P. Hawk [Alonzo P. Hawk], who schemes to close Medfield College and push a fraudulent glass-claim partnership.
At home, Ned's wife, Betsy Brainard, is overwhelmed by financial stress and increasingly unsettled by the attention Ned receives from an old high school flame who reappears in their lives as Shelby Ashton [Shelby Ashton]. The tension intensifies as Shelby tries to rekindle the past, testing the boundaries of Ned and Betsy’s marriage. When Ned senses the threat Shelby poses, he uses the weather ray to rain inside Shelby’s car, with thunder and lightning, turning the interior into a flooded mess and triggering a crash that mirrors a past mishap from another chapter of his life.
Flubbergas also proves its odd usefulness when Medfield rides to a football victory over Rutland, buoyed by the crowd-pleasing oddity of the weather-influenced win. Betsy, meanwhile, makes a decision about her loyalties, and Ned is arrested for damages linked to the glass incident, straining their relationship even further.
Inside the courtroom, the prosecutor presses Ned to abandon his experiments, painting a bleak future if he continues. Yet the county agricultural extension agent steps forward with a surprising testimony: crops around town have thrived because of Ned’s work, under the label Dry Rain. This new evidence shifts the momentum, and Ned earns an acquittal, allowing the couple to reunite.
The final stretch of the story follows them home in their flying car, where Betsy expresses a renewed fondness for Ned’s science, and the pair share a kiss. The film closes with a whimsical image: a football filled with Flubbergas soaring into outer space, a playful nod to the film’s scientific whimsy and its ties to Ned’s inventive spirit.
