In the sweltering Texas heat of 1998, the lives of Beavis and Butt-Head spiral out of control when their high school science fair experiment goes disastrously wrong, leaving nothing but ashes in its wake. In a desperate attempt to steer these wayward boys back on track, a judge sends them to space camp at the Johnson Space Center, where they unexpectedly develop a fascination with a docking simulator. Their remarkable skills capture the attention of NASA astronaut Captain Serena Ryan (actor name), who invites them to train as astronauts. Misunderstanding this golden opportunity, they tragically think it’s all about indulging their immature desires as they set out on an unexpected journey to space.
As Beavis and Butt-Head launch into the vast unknown, they unwittingly leave chaos in their wake. When Captain Ryan prepares to make a selfless sacrifice, they misconstrue her heroism as an invitation for intimacy. Eager to join the action, they suit up, unaware that Serena is preparing for a serious mission. Mistaking her focused demeanor for flirtation, she angrily takes control of the Space Shuttle Endeavour, launching them into the depths of space, where they collide with a micro black hole.
Suddenly, the boys find themselves emerging in Galveston, Texas, two decades later, in 2022. The Pentagon quickly takes notice of their unusual arrival and starts tracking them. Meanwhile, Captain Ryan, who is now the Governor of Texas, is determined to eliminate any trace of her past mistakes, desperate to cover up the incident. During their wild journey, they meet smarter versions of themselves from an alternate universe, Smart Beavis (actor name) and Smart Butt-Head ([actor name]), who inform them about a looming threat: they have just two days to reach a portal atop Mount Everest to save the multiverse from impending destruction.
As they stumble through this new reality, Beavis and Butt-Head accidentally activate Siri on an unfamiliar iPhone, inadvertently mistaking her for Captain Ryan. Beavis develops an infatuation with the voice assistant, convinced she is waiting for him in Highland. When Siri suggests creating a smart home, the duo believes they are in for a romantic evening with Captain Ryan. However, their misadventures land them trapped in a port-a-potty, from which Beavis finds himself transported to a university campus. Smart Beavis and Smart Butt-Head implore them to enter the portal, now relocated nearby, but instead, they wander into a gender studies lecture, where the professor's discourse on white privilege confuses them into thinking they have a free pass for mischief.
Ultimately, they steal a police car, only to be arrested. Inside jail, Beavis takes some drugs offered by a fellow inmate, triggering his transformation into the wild alter ego Cornholio ([actor name]), which leads to a major riot. When the warden discovers the boys were believed to have died back in 1998, he mistakenly concludes that they are angels and sets them free.
Returning home after all their escapades, they find their house listed for sale, which leads to betrayal and confusion among them. Miscommunication drives a wedge between the friends, but their separation is short-lived when government agents snatch them away without explanation.
In a surprising turn of events, Jim Hartson, Serena’s beleaguered lieutenant, secretly helps the boys escape, revealing his knowledge of the events that transpired. The duo is pushed to reconsider the mysterious portal, but they resist, preferring to deal with their own mess first.
As all factions converge at Beavis and Butt-Head's home, an embittered Serena prepares to confront them. However, she soon realizes they are merely misguided teens, not the threats she imagined. Amidst the chaos, Smart Beavis and Smart Butt-Head urge their less intelligent counterparts to tackle the portal crisis.
The climax unfolds dramatically as Hartson is inadvertently pulled into the rift, effectively sealing it and averting the destruction of the multiverse. While Beavis attempts to share his feelings with Serena, Smart Beavis steps in, eager to take her on an expedition through the cosmos alongside Smart Butt-Head.
In a twist ending, the government, aiming to retain secrecy, rewards Beavis and Butt-Head by restoring their house to its former glory. In an alternate dimension, a gathering of different Beavises and Butt-Heads celebrates Smart Beavis's groundbreaking accomplishment: being the first of their kind to experience the thrill of intimacy.
