In a tense opening, Colton West—an A-list actor who often commands attention—has a heated disagreement with Darren, the film's director, and storms off in his car. An earthquake rocks the city as the Santa Monica Mountains erupt, hurling volcanic bombs and debris into the air, and a massive, fire-spitting spider bursts from the car in front of him. Colton swerves and speeds away, fighting through traffic to escape the chaos.
Back home, he returns to tell his wife Olivia West what he witnessed and to urge immediate departure. He grabs a shotgun from the house, grips the wheel, and heads off to find his son, Wyatt West, who had left the house under the pretense of a soccer game.
Meanwhile, Wyatt and his friends—Jordan, Eli, and Travis—are investigating the explosion when a sinkhole opens up, spewing spiders that spit fire. The teens flee to a warehouse to hide, but tragedy strikes as Eli is captured and killed by a spider; Jordan suffers burns and then dies, with baby spiders crawling from her as she falls.
Colton's journey continues as his car is damaged by a lava sinkhole; he climbs out and spots a crowded tour bus up ahead. He masterminds a ruse to hijack the bus, tricking the driver and taking control. A spider leaps onto the moving vehicle; he brakes hard, the spider creature falls off, and he runs the bus off the road, yet the bus's front quickly dissolves due to the spider's acidic blood. He halts the vehicle and evacuates the passengers just before it detonates.
Back at Olivia's house, a spider slips in through the chimney, but she shoots it dead. The army arrives and begins to move in; they ultimately manage to rescue Olivia from her home, and she then boards an army truck to join the effort.
Colton teams up with his film-world buddy, the stuntman Pirate Jack—Pirate Jack—and the two set out along the road. Their car is punctured by a lava-vein, forcing a pivot to safety. Spiders flood from more sinkholes and chase them into a museum. Inside, they meet a scientist who studies the spiders and volcanic activity. He theorizes that the spiders were buried in magma millions of years ago and have resurfaced because the volcano is waking up. The scientists conclude that the only way to stop the eruption is to kill the Queen spider.
The group navigates the museum as the situation grows more dangerous. A spider claws its way into the warehouse where Wyatt and friends are sheltering; Jordan's death is confirmed, and her baby spiders attack Travis, setting him on fire. Wyatt climbs to the roof to reach a phone signal and texts Colton his location.
The army truck with Olivia is attacked by spiders; she hides under a fire blanket as the arachnids breathe fire inside. The soldiers are overwhelmed and killed, but Olivia manages to kill a spider and escape.
Meanwhile, the museum becomes a battleground as Jack is killed and Colton escapes. Olivia finds Colton, and the pair set out to rescue Wyatt.
Wyatt is cornered on the warehouse roof by a relentless spider; Olivia and Colton arrive and crush it beneath a passing Army truck. Colton realizes that liquid nitrogen could be the key to stopping the invasion. They head to a nearby special effects store to regroup with Colton’s colleagues from the film set.
With a clear plan, Colton devises a nitrogen-based attack. He and his team plant bombs around the queen spider’s underground chamber and trigger detonations as the spiders spill from the sinkholes. The army fights off the onslaught while the queen spider finally emerges. Using a jet pack from the film set, Colton soars toward the queen and drops a bomb of liquid nitrogen into the spider’s mouth, a dramatic strike that neutralizes the creature and saves the city.
