We open with the credits displayed over a spider web; we zoom in closer to the spider in the center. It is evening and a pickup truck drives down a lonely highway. Jack Flynn (Merritt Stone) is driving, he smiles and examines the birthday present he got for his daughter. The box contains a handwritten note, "To Carol with Love, Dad" and a bracelet. He notices something on the road ahead, screams and recoils. A rope-like material hits the windshield of his truck and his face is covered in blood. We hear the sound of the crash.
It is morning in the small town of River Falls. Carol Flynn (June Kenney) is walking to school. Her boyfriend, Mike Simpson (Eugene Persson) joins her. He gives her a small, wrapped birthday gift. She acts detached. She tells Mike her Dad didn't return home last night. They have a silly argument and she returns his gift, unopened. The school bell rings.
Professor Art Kingman (Ed Kemmer) is performing an experiment in class. He is lecturing his students, on electricity. Mike and Carol are in his class. They pass a note back and forth and she takes her gift back. She asks Mike if he will ask Joe to borrow his car so they can look for her Dad. Mike asks Joe (Troy Patterson) and he agrees. They drive along the most likely road her Dad would have taken the night before and discover a thick, rope-like material attached to a tree. Mike notes the material is sticky and finds a section of auto windshield on the highway. Carol spots a box on the shoulder of the road. She opens it and finds the note in her Dad's handwriting. She glances down a ravine and spots an old truck. They walk down to get a closer look, verify it belongs to her father, but find no sign of him. Glancing over they spot a cave entrance and walk over to it. As Carol looks around, Mike retrieves a battered and shredded hat. As Carol approaches, he hides it behind his back and then drops it out of her view. Mike enters the cave with Carol close behind. Carol calls for her father, progressively louder, until it loosens and dislodges a stalactite that almost hits the pair. They wander a little further and discover the skeletons of two humans. Carol falls though an opening with Mike close behind. They land on a woven net of sticky material, the same material they found on the road. They hear a strange sound, a screeching sound followed by the appearance of a huge tarantula. They slip though the web and run for the exit. The spider gives chase then stops and turns around.
Kingman is examining the rope; he concludes that it appears to be a large piece of silk. Carol and Mike are present in Kingman's living room, along with Kingman's wife, Helen (Sally Fraser) and Mike's dad, Mr. Simpson (Hal Torey). Kingman finds the story a bit incredible, but recommends the Sheriff be notified. Mike informs his teacher they did. Mr. Simpson adds that the Sheriff did not believe their story. Kingman is concerned enough to phone the Sheriff himself. Sheriff Cagle (Gene Roth) is playing checkers with his deputy Pete Sanders (Bill Giorgio) when the phone rings. Kingman appeals to the Sheriff via the missing Mr. Flynn story as opposed to the giant spider story. Cagle agrees and forms a posse to search for Flynn. Kingman also suggests the Sheriff call a pest control company and bring plenty of DDT. They arrive at the cave. The pest control operator (Bob Garnet) suggests a two to four percent solution for spiders, but Kingman insists on a fifty percent solution. Mike and Carol are present to advise the Sheriff, along with Kingman and Mr. Simpson. They walk down the ravine, past the truck and into the cave. Cagle leads the group asking Mike and Carol for directions. Carol screams, which brings the group running. They find her sobbing in Mike's arms. The source of her anguish is the desiccated remains of her father. They wrap the body and remove it from the cave. The Sheriff and Kingman continue searching the caverns and find the web. Cagle orders the DDT be deployed. They don gas masks and start spraying while sitting on the top of the web. The spider approaches. The sheriff and his deputy fire their rifles, but as Kingman had warned them, insects have a simple nervous system; you can plug holes in them all day long and not hit a vital spot. Deputy Sheriff Pete Sanders is killed by the spider before one of the Sheriffs bullets wounds it. They exit the cave, but Carol, checking her pocket, notices the box missing. She must have dropped it in the cave. Kingman tells her she can't go back, as the cave is now full of gas. Mike tells her they will come back later to retrieve it. Cagle tells Sam Haskell (Mickey Finn), the Road Foreman, to make arrangements to seal the cave entrance. Kingman tells the Sheriff he wants the spider for scientific investigation, but Cagle wants no part of it. Kingman makes his own arrangements to move the spider temporarily to the High School. He tells the photo club instructor, Mr. Fraser (Jack Kosslyn) that he will send photos and a report to the State University. His main concern is how it got so big. Kingman and Fraser get closer; Kingman identifies it as a Bird Spider. It moves and strikes Fraser, knocking him to the floor. Convinced its alive, Fraser asks why it kicked him. Kingman assures him that it is dead and only experienced a muscle contraction--Galvani's reaction.
Carol is sobbing on the couch when her mom comes down from upstairs. Mrs. Flynn (June Jocelyn) tries to comfort her. She tells Carol that Mike called. Carol calls the local movie house where Mike works for his Dad, the owner. She asks Mike to take her back to the cave so she can find her bracelet. She pleads and he relents. He makes arrangements with Joe, again, to borrow his car. Joe and his friends need to rehearse but the Recreation Room at the High School is locked. The spider is being stored in there until that evening when representatives of the State University come to retrieve it. Joe tells Sam, the base player (Skip Young) they'll appeal to Hugo the Janitor (Hank Patterson) to let them in to use the Rec Room. Hugo reluctantly agrees, but warns them to leave the spider alone, "...do not monkey with that thing." He unlocks and opens the door and they all gather around it. Sam expresses his discomfort, but they get down to rehearsal. Before they can start, a crowd of students enters the Rec Room through the open door. They start playing and the students start to dance. The noise reactivates the spider from its torpor. The kids scatter, leaving Hugo to lock the door on the spider. He goes to a payphone to call Kingman at home. As Kingman answers, Hugo screams and drops the phone just as the spider breaks through the wall and kills him. Kingman calls the Sheriff.
Mike and Carol return to the cave, unaware the spider is on the loose. Kingman meets Cagle in front of the High School as the spider emerges. The spider zeroes in on Cagle, but Kingman picks him up in his car and they escape. The spider strolls through town amid a panicked population. Back in the Sheriff's office, Kingman and his new deputy, Dave (Bob Tetrick) wait for Cagle to finish his call for help to the state capitol, but the long distance lines are down. Deputy Dave volunteers to take his motorcycle to a nearby town to call for help. Jake (Howard Wright) drives through town with his belongings in and on his car. Cagle asks why he is evacuating. He is told, "The spider ran me out of house and home...Its down on Maple Street." That is where Kingman lives, and he fears for the safety of his wife and infant son. Helen is at home feeding the baby when the spider walks up to the house. It breaks a window and blocks her escape. Kingman drives his car into it. It chases after his car back to the cave. Kingman ditches it and drives back home to a wrecked house, but his family is safe.
Mike and Carol are searching the cave with no luck. Carol finds an opening and speculates that the bracelet may have fallen through. They make their way down and Mike finds it. He insists he hold it until they exit the cave. Unfortunately the spider returns to the cave, but Jake sees it and drives back into town to report same. Sheriff Cagle is waiting patiently in his office with Mr. Simpson and some of his posse when Kingman brings in the desiccated body of Deputy Dave he found in town. He never got to Springdale to summon help. Jake arrives and tells Cagle the spider is back in the cave. Kingman suggests to Haskell, the Road Foreman, they use dynamite to seal the cave entrance. Cagle deputizes Mr. Simpson, leaves him in the office, and takes everyone else out to the cave.
Joe calls the Sheriff's office and Mr. Simpson answers. Joe tells Simpson that Mike borrowed his car and hasn't returned it. Simpson calls Mrs. Flynn to inquire as to their children's whereabouts. Mike and Carol are now lost in the caverns. They wander around. As they snack on a candy bar, they hear the spider approaching and run. Sheriff Cagle and his posse arrive at the cave entrance and begin to unload the dynamite. Mr. Simpson and Mrs. Flynn are driving to the cave to warn the Sheriff that the kids might be in the cave. Mike and Carol find the web and the pathway out of the cave. They get stuck. Simpson and Mrs. Flynn arrive and find Joe's car just as Haskell dynamites the cave entrance. It causes a partial collapse of the cavern Mike and Carol are occupying. Simpson tells Cagle the children are in the cave and they will have to reopen it. Haskell and his men begin excavation. Kingman suggests, as a defense, they try electrocuting the spider.
Inside the cavern, Mike and Carol are covered with rocks and dirt. They are shaken but not seriously hurt. The excavation crew reaches bedrock and uses dynamite to create an opening. Mike and Carol make their way to the sounds of the crew, but the spider is back on the prowl. Kingman briefs the power line foreman (David Tomack) on his plan for using high voltage. The kids try to find a safe place in the cavern, but are trapped on a ledge with the spider in pursuit. The excavation crew breaks through and sends down a ladder. Kingman descends with the power cables and electrodes. He calls to Carol and Mike and follows their screams for help. The spider crawls down the cavern wall towards the ledge they occupy. The spider is now between Mike and Carol on one side and Kingman and Cagle on the other. Cagle tosses one of the electrodes over to Mike, while Kingman holds the other. They energize the line. The spider is caught between the two electrodes and an arc of electricity passes through it. It shrieks in pain and falls. Cagle and Kingman rescue the kids and Carol and Mike are reunited with their relieved parents. When everyone exits the cave they reseal it with dynamite again. We close with a medium shot of the spider impaled on a stalagmite.