In the tranquil mining town of Prosperity, Arizona, an unexpected incident involving a rabbit triggers a disaster when a barrel of toxic waste inadvertently contaminates a nearby reservoir. An exotic spider farmer named Joshua is a frequent visitor to the site, diligently collecting crickets for his various spiders. Despite the toxic environment and the spiders having ingested the pollutants, Joshua remains blissfully unaware as the arachnids appear unaffected. During one of his visits, he introduces the local boy, Mike, to his collection, showcasing a diverse mix of spiders, including Jumping spiders, Trapdoor spiders, Tarantulas, and a prominent female orb-weaver he affectionately names Consuela. However, Joshua's fate takes a dreadful turn when an escaped tarantula bites him, causing him to knock over his spider cages. Tragically, he meets his end at the fangs of the very creatures he tended, resulting in their rapid growth to enormous sizes.
As Mike makes his way home, he confronts his mother, Sheriff Samantha 'Sam' Parker, and Deputy Pete, who are seen extracting the toxic barrel from the pond. The Mayor of Prosperity, Wade, convenes a town meeting in the local mall to discuss the proposition of selling the mines and relocating. Meanwhile, Chris McCormick (David Arquette), whose father was once the owner of the mines before his passing a decade ago, emerges as an opposing voice against Wade's plan. As a subplot, Chris begins to develop a romance with Sam.
In the midst of the chaos, Harlan Griffiths (Doug E. Doug), the town's quirky radio DJ and UFO enthusiast, broadcasts a theory claiming that the increasing number of missing pets in Prosperity is linked to extraterrestrial abduction. His bizarre claims are met with skepticism and dismissed as mere flights of fancy. Undeterred, Mike ventures out to locate Joshua but instead finds the spider farm encased in webbing and Joshua's lifeless body. Spotting a massive spider shadow lurking in the mines, Mike urgently warns Chris about the spiders' unnatural growth, but his concerns are initially met with disbelief. As the mines reopen and the miners search for a legendary gold strike, a miner soon becomes the first victim of a giant spider lurking within.
The arachnid menace spreads throughout Prosperity, invading numerous locations. Deputy Pete's cat is dispatched by one of the spiders in the air ducts, while Wade's farm sees the aggressive assault of Trap Door Spiders against his ostriches. Amid personal turmoil, Ashley breaks up with her boyfriend, Bret, who then encounters a swarm of jumping spiders, resulting in his isolation and perilous circumstances in the mines. As the chaos escalates, Chris discovers that his Aunt Gladys and her dog have also been taken by a male orb-weaver in their basement. Initially dismissive of Chris and Mike’s terrifying tales, Sam begins to realize the severity of their warnings when she witnesses a giant male orb-weaver trying to abduct Ashley (played by Scarlett Johansson) along with Chris and Sam ultimately springs into action to save them.
When the situation spirals out of control, Sam instructs Pete to gather every available weapon from the police station as they seek refuge in Harlan's trailer, which houses an operational radio station. As they send an urgent message over the airwaves about the spider apocalypse, a colossal tarantula, dubbed the "tank" of the spider horde, launches an attack on their trailer, prompting a precarious escape. With the town besieged by a ruthless tide of spiders, Chris desperately attempts to sway Harlan into believing the spiders are "from Earth," instead resorting to declaring they are “Spiders from Mars” to suit Harlan’s whimsical mindset. Sam organizes an evacuation to the mall, where Chris, Sam, Mike, Ashley, Harlan, Wade, and other townsfolk barricade themselves as they prepare for the impending spider assault. In a gripping scene, Harlan and Chris scale the roof and radio mast to seek help from the U.S. Army, only to be mistaken for pranksters. Harlan’s daring jump from the roof ultimately leads him to reunite with Pete, allowing them to team up against the growing spidery menace as Chris heads to the basement where the remaining townsfolk are trapped.
In a dramatic turn of events, Bret arrives with a forklift that breaks down the barricade, allowing everyone to navigate through the mines. Before long, Chris finds his Aunt Gladys and the gold his father sought but not without facing Consuela. Using a clever diversion with perfume, he narrowly escapes and triggers an explosive finale by igniting the methane gas that fills the mines. In the aftermath, police finally arrive on the scene, initially believing they are dealing with an extraterrestrial issue rather than a spider crisis. However, Pete dispels the misconceptions, revealing their true nature—ordinary spiders that have been poisoned by the toxic waste.
In the closing moments, Harlan's radio broadcast sums up the chaotic events while hinting at a possible cover-up by the town officials, even as he continues his transmissions. He reveals Chris’s plans to reopen the gold mines, but wryly notes, "that is another story altogether." As the camera zooms in on Harlan's mouth, it beads with irony, showcasing his new three gold teeth as a souvenir of the bizarre, harrowing adventure.
