Stanley Stupid, Tom Arnold and his wife Joan Stupid, Jessica Lundy, believe they’re repeatedly targeted by a vast conspiracy that steals their garbage every week. To uncover the truth, Stanley tails the garbage truck all the way to the city dump, where he runs into Colonel Neidermeyer, [Mark Metcalf], a former military man who’s selling contraband weapons to terrorists. Mistaking Stanley for a secret agent who has uncovered their operation, Neidermeyer orders him assassinated. After a string of failed attempts that somehow end with Stanley inadvertently eliminating his would-be killers, he narrowly escapes a car bomb and is presumed dead by Neidermeyer.
Meanwhile, the Stupid kids—Petunia Stupid, [Alex McKenna], and Buster Stupid, [Bug Hall]—believe their parents have been kidnapped by a Chinese restaurant and head there to search for them. They eventually reconnect with Joan, who reveals her theory that the police have turned against the family and are somehow responsible for Stanley’s disappearance.
Back at home, Joan explains Stanley’s last job for the postal service, where he uncovered a trail of letters marked “return to sender,” and he was fired before he could discover who “Mr. Sender” actually was. He shares with his family the conspiracy he’s weaving from their wild imaginings, including the idea of an Evil Mr. Sender who intends to steal all the mail and use the police to kidnap anyone who uncovers the plot. When they locate local museum curator Charles Sender, [Robert Keeshan], in the phone book, they set off to pursue him to a television studio.
In the studio, Stanley appears on a talk show, and is spotted by Neidermeyer, who abducts him and takes him to an army base. While a hostage, Stanley overhears the address of the warehouse where the illicit arms deals are taking place. After escaping, the Stupids rush to the warehouse to confront Sender and recover the stolen mail. A heated confrontation unfolds with Neidermeyer and his terrorist associates, causing explosions that draw the police’s attention. The criminals are arrested, but Neidermeyer manages to slip away.
The would-be heroics take a comic turn when Stanley offers what he believes to be righteous advice about repentance, but Neidermeyer reads it as directions. The family returns home to a tense standoff, only to be saved at the last moment when a deliveryman—whom Joan had arranged earlier—knocks Neidermeyer out at the front door. In a finale that blends chaos and warmth, the Stupids celebrate their unlikely victory with an outdoor barbecue, reaffirming that their quirky, imperfect teamwork keeps them together despite the chaos around them.
