NOTE: This spoiler was submitted by Jeremy

The film starts with Beavis and Butt-Head (both Mike Judge) as they fly a spacecraft through a Death Star-like structure. They are getting shot at but they manage to fly away, despite causing a ton of destruction.

The guys are then shown in their living room on their couch, telling the viewers that it has been a while since they have been seen, so they recap what brought them to this moment. It begins in 1998 at their high school in Highland, Texas. Butt-Head is trying to see how many times he can kick Beavis in the nads before he passes out. Their teacher then convinces them to join the school’s science fair, because the winner gets a free trip to space camp. The boys find a robot foot, and Butt-Head gets an idea for his “experiment. He winds up the leg and uses it to kick Beavis in the nads so hard that he goes shrieking and flying all across the gym. This causes them to break the other experiments and start a fire.

The boys are taken to court, where the judge decides that they should be sent to space camp for eight weeks. There, Beavis and Butt-Head meet the program’s director, Captain Serena Ryan (Andrea Savage) and her less-respected colleague Jim Hartson (Nat Faxon). They demonstrate a docking simulator, which the boys become fascinated with because it looks sexual. The two spend 18 hours playing with it and using the robot hand to make it look like it’s jerking off the shuttle. Serena and the other superiors are impressed, thinking Beavis and Butt-Head are qualified to join them in docking their shuttle into the space station Mir as they prepare to study a black hole, but from Serena’s wording, the boys think she is propositioning them for sex. They promptly agree.

Beavis and Butt-Head go through extensive training for the mission. Before they join Serena and her team, Beavis gives an impassioned interview about how this has always been his lifelong dream (to “score”), which everyone thinks means he has always dreamed of being an astronaut. The team blasts off into space. As Beavis and Butt-Head await to score with Serena, they start screwing around with the controls and look through a telescope right into the sun, temporarily blinding them. When the time comes to dock the shuttle, the boys mess around even more and cause both the destruction of the shuttle and the space station.

The team makes contact with NASA, where they learn they have barely any oxygen or fuel left to return to Earth. Serena determines that there is enough oxygen for five of them, and there are seven of them in there, so two people would have to sacrifice themselves. While everyone wants Beavis and Butt-Head to do it, Serena decides to be the one to do it since she knows much of the blame for bringing two idiots onboard will fall on her. As she prepares to head out, she tells Beavis and Butt-Head to watch her through the window as she goes off, and she gives them her grandmother’s bracelet to encourage them to fulfill their dreams. The boys once again misread her words as watching her through the window to undress. They get in their space suits and step outside to do so. Serena, having had enough of them, uses the robot arm to whack the boys further out into space and pretend to the rest of the team that they willingly sacrificed themselves. The boys float until they find the black hole and get sucked into it.

Beavis and Butt-Head land in Galveston in 2022. They are then met by alternate version of themselves that are actually intelligent, so they are known as Smart Beavis and Smart Butt-Head. They try to explain to the main Beavis and Butt-Head that there are infinite versions of themselves across the multiverse, but in two days time, they need to jump into a portal back to 1998, or their realities will collapse upon each other. At the same time, the Pentagon finds readings of both regular Beavis and Butt-Head plus their smart versions arriving on Earth.

The boys continue walking and find that Serena is now governor of Texas, and they volunteer for her re-election because they think it will better their chances of scoring with her. When she learns about their return, she is worried that they will tell everyone that she tried to kill them, costing her her political career.

A tourist family asks Beavis and Butt-Head to take their picture, but they think that the family’s smartphone is a small TV, and they cause the family to fall off the pier and into the ocean. The boys then learn from a nacho vendor that they can pay with their phones, so they go on a shopping spree and mostly buy a ton of nachos. They book a hotel room, and Beavis talks to Siri on the smartphone, thinking it’s Serena giving herself a nickname. While Beavis continues to “talk” to her, he expresses feelings that he doesn’t understand Butt-Head sometimes. After Siri asks if Beavis would like to set up his home, he interprets this as Serena being at their home in Highland, so the boys leave the hotel room and head off before the Pentagon agents track them down.

Beavis and Butt-Head walk until Beavis uses a port-a-potty and drops the phone in there. He gets stuck trying to get it out, and while Butt-Head tries to help (or rather, get Beavis stuck even more), the port-a-potty is picked up, and the two unknowingly spend the evening being transported to another town. They end up at a college campus and are found again by Smart Beavis and Smart Butt-Head. They tell the boys that they moved the portal to 1998 behind one of the college buildings that nobody would ever think to look behind. Beavis and Butt-Head then enter a gender studies class, which has them get lectured after they use the term “slut” to describe Serena. The professor (Tig Notaro) explains to the class and the boys that this is an example of white privilege, and the boys become amazed that they can get away with it. Unfortunately, the boys abuse it (naturally) and try to steal a cop car, which gets them arrested.

In jail, the boys try to figure a way to get themselves free. A prisoner tells Beavis to smuggle pills in his pants, but he instead starts wolfing them down like candy. This causes him to turn into Cornholio, and as he yells that he needs “TP for his bunghole”, the other prisoners take this as a means to riot against the guards and start an uprising for their unfair treatment. The warden then learns that Beavis and Butt-Head don’t have records because they were declared dead in 1998. They let them go and give into the demands of the prisoners for more toilet paper. Serena and Hartson show up too late and try to pardon them, thinking they are still there, but they end up taking two large prisoners who tie them to a tree and carjack them.

Beavis and Butt-Head take a bus back to Highland and find that their house has been renovated after Beavis’s mother sold the house following his supposed death. When they find that even their TV is gone and that Serena isn’t there, the two have an argument where they blame each other for their predicament and split up. As the two walk away, they quickly miss each other, but they are promptly captured by government agents.

Serena and her team interrogate the boys in a motel room, but they get nowhere because of how stupid the two of them are. Hartson frees Beavis and Butt-Head because he has evidence of what Serena tried to do to them, and he is ready to expose her because he is sick of being put upon by her. The boys then steal a car and start driving back to their house, while the agents pursue them. Smart Beavis and Smart Butt-Head then attempt to get the boys to head toward the portal and show them the lives that they would live if they go back to their time, but it’s just the two of them older and fatter in their house, watching their younger selves through a portal and thinking that they are on a TV show, and vice versa.

Everyone makes it back to Beavis and Butt-Head’s house. The agents draw their guns on the boys, and Serena gets ready to eliminate them. Beavis and Butt-Head then continue to assume that they are finally going to score, which leads Serena to realize that they are not her enemies and were never out to get her, they are just stupid, horny teenagers. Smart Beavis and Smart Butt-Head then try one last attempt to get the boys to go through the portal because reality is collapsing. Hartson then tries to step in and have Serena exposed, but he accidentally gets sucked into the portal, and everything goes back to normal. Before the agents leave, Beavis tries to confess his feelings to Serena, but Smart Beavis beats him to the punch and promises to show Serena the infinite wonders of the universe, in exchange for sex. Serena reluctantly agrees and joins Smart Beavis and Smart Butt-Head.

Beavis and Butt-Head conclude their tale, now having moved back into their old house and having gotten a new TV. Meanwhile, Smart Beavis and Smart Butt-Head join the other Beavis and Butt-Heads from multiple universes. Smart Beavis is praised for being the first Beavis of them all to finally score, while Smart Butt-Head is commended for watching in a closet.

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Dim-witted duo Beavis and Butt-Head get sent to space camp after destroying their school gym during a science fair. Astronaut Captain Serena Ryan thinks the two are qualified to help them dock their shuttle into a space station as they prepare for a mission to study a black hole, but the two are just fascinated by how sexual the docking simulator looks, and they also believe Serena is propositioning them for sex. Naturally, their antics cause destruction to the shuttle and space station, so Serena has them booted off the shuttle, where they are sucked into a black hole.

The boys end up being transported from 1998 to 2022. Alternate versions of themselves - Smart Beavis and Smart Butt-Head - try to get them to pass through a portal back to their time within two days, or reality will collapse upon itself. Serena is now governor of Texas and believes that the two have come back to expose her for trying to kill them, but they are just still looking to score with her.

After a series of misadventures trying to make their way back to their house in Highland, Beavis and Butt-Head have a fight and split up until they are captured by Serena and her agents. Her put-upon lieutenant, Jim Hartson, frees the boys and wants to expose Serena for her crimes. Everyone makes it back to the boys' house, where Serena comes to realize that they are not out to get her, they just want to have sex with her. Smart Beavis and Smart Butt-Head open the portal for the boys to go back to 1998, but Hartson accidentally gets sucked into it, thus saving the world.

The boys go back to their normal lives, while Smart Beavis becomes the very first Beavis in the multiverse to score when he gets with Serena.