Plot

An absent-minded professor discovers "flubber," a rubber-like super-bouncy substance.

Having already missed his wedding to his beautiful sweetheart, Sara, twice, the happy, absent-minded professor, Dr. Phillip Brainard, is struggling to come up with a brilliant invention to save Medfield College from closure. However, as good things come to those who wait, the eccentric inventor stumbles upon an innovative but highly unstable formula for "Flubber", a mysterious rubber-like hybrid elastomer that can lift objects, bouncing and stretching as if it had a mind of its own. Of course, this unique but untamed creation catches the attention of a ruthless rival who is bent on getting his hands on the extraordinary substance. But first, he will have to catch it. Can Dr. Brainard protect his invention?—Nick Riganas

Phillip Brainard, who is a professor at a college and a small-time worker in his own basement, is trying to create a great invention. Unfortunately he always failed to invent something, but he is so obsessed about it that he even puts off his own wedding to his fiancée, Sara. One day, he finally invents something that works; this substance can cause even the heaviest things to bounce in the air, 'flying rubber', but Brainard called it "Flubber". However, he sees Sara going out with another professor from another college and several criminals trying to rob him. So, Brainard has to find out a way to defeat the criminals and to get back his Fiancee. Hopefully, Flubber can help him.—John Wiggins

Professor Philip Brainard (Robin Williams), a professor at Medfield College, is developing a new energy source in an attempt to raise enough money to save the college from closure. His occupancy with his research distracts him from his fiance and the president of Medfield College Doctor Sara Jean Reynolds (Marcia Gay Harden). Philip's house is a junkyard, full of waste materials from his failed experiments in the past. Philip works on his research regarding the interaction of matter and energy. To save time, Philip has all sorts of machines in the house to prepare his morning coffee and breakfast.

Philip has missed two weddings in the past as a result of this, much to the anger of Sara. On the day of the third attempted wedding, Philip is approached by his former partner Wilson Croft (Christopher McDonald), who has profited from ideas he has stolen from Philip and now desires to steal Sara from Philip and make her his wife, which he declares directly to Philip.

While preparing for the wedding, Philip has a breakthrough with the help of his drone robot assistant Weebo (Jodi Benson), who is secretly in love with him. Weebo has the power to hover and move around the house with his flight capabilities. Weebo had intentionally tried to hide the calendar information that Philip is supposed to be marrying Sara at 6 PM that day, but Philip met Sara in college who reminded him that the wedding was at 6:30 PM that very day. Sara is clear that this is the very last time she would try to marry Philip.

Philip notices a gap in his polymer chain and fixes it on his computer and gets a stable compound. Philip gets excited and runs to his lab in the basement to synthesize the compound. His experiment results in a sentient green goo with enormous elasticity and kinetic energy, which wreaks havoc on the neighborhood before the professor recaptures it. Weebo classifies the substance as "flying rubber", leading Brainard to christen it "Flubber". Flubber is an Elastomer, with high viscosity, can phase shift, mold-able, ticklish, fold-able, ductile and so on. Flubber has incredible inertia, and Philip can only stop it with a baseball glove.

Meanwhile, Sara gives up at the church after waiting for Philip all evening and is met by a sympathetic Wilson.

Philip continues to experiment with Flubber and discovers that Flubber can be controlled with a steady stream of Gamma particles. Philip discovers that as it bounces, Flubber increases in speed rather than decreasing. After he relays this information to Weebo, she informs him that he has missed the third wedding. Philip realizes too late that it is already the following morning, and he had missed his chance with Sara. A heartbroken Sara refuses to hear his explanation, leaving Philip determined to prove Flubber's worth and win her back.

Meanwhile, Medfield College sponsor Chester Hoenicker (Raymond J. Barry), who is threatening to close the school, is unhappy that Philip flunked his son Bennett (Wil Wheaton) in chemistry class. As a result, Bennett was kicked off the basketball team and put on academic probation. That night, Chester sends his two security guards Smith (Clancy Brown) and Wesson (Ted Levine) to Philip's house in an attempt to persuade Philip into giving Bennett a better grade.

When they get there, they spy on Philip testing the Flubber and are suddenly knocked out unconscious by a Flubber-coated golf ball and bowling ball. Smith and Wesson return to Chester and report about what they saw.

Philip uses Flubber to give his car the ability of flight. During a test run, Philip overhears Wilson make a flirtatious bet with Sara about Medfield's upcoming basketball game against Rutland. Wilson says that if Sara's college loses, she would have to the mountains for the weekend with him.

Afterwards, Weebo attempts to confess her love of Philip when he is depressed about seeing Sara with Wilson, only to be shrugged off as a computer. In response, she secretly creates a holographic human version of herself named Sylvia in hopes of winning him over. Before Weebo can make out with Philip in this form as he sleeps, Philip awakens with another idea for Flubber. He enters an empty basketball court and tests the effects of Flubber on a basketball.

Later, he gives Flubber-padded shoes to the unskilled Medfield basketball team to increase their abilities allowing them to bounce incredibly far, and enabling them to beat Rutland, but his attempt to win back Sara fails.

Back in Philip's home, he had left Weebo in charge and to keep Flubber under its containment structure at all times. A spiteful Weebo unleashes Flubber from his case, allowing him to dance around the house and cause general mayhem. After the close but successful basketball game, Philip's attempt to win Sara back into his favor fails. Philip dumps all of his emotional baggage onto Weebo, saying his absent-mindedness is due to his love of Sara.

Weebo, putting the professor's happiness before her own, shows Sara footage of Philip's declaration, and the couple reconciles. Philip demonstrates Flubber's abilities to Sara and they discuss how he can be used for profit. However, Chester discovers Flubber's existence and offers to buy it and forgive the college's debt. Philip and Sara refuse, making a deal with the Ford Motor Company instead and saving the school.

Chester sends Smith and Wesson to break into Philip's house and steal Flubber. Weebo attempts to fend off the goons, only to be struck down with a baseball bat. Philip and Sara return home and find that Flubber is gone and Weebo is destroyed. Philip discovers that Weebo created back-up data of herself onto Philip's computer in the event of her demise, her "daughter" Weebette.

Philip and Sara confront Chester under the guise of selling him additional Flubber, and discover he is in league with Wilson. Unleashing Flubber, Philip and Sara defeat Wilson, the Chesters, and their henchmen. Sometime later, Philip saves Medfield from closing with Flubber and the happy couple finally have a successful wedding and embark on their honeymoon to Hawaii in the flying Thunderbird with Weebette and Flubber.