During the summer, an alien race lands on Earth and targets a secluded college town. There, an extraterrestrial team, consisting of Doctor Kozmar (John Carradine), Doctor Zarma (Julie Newmar), and their associate, Cora (Tina Louise), occupy an empty hospital in order to extract blood platelets from humans. The donor blood will prevent bone degeneration among the aliens and prolong their lives by 100 years or more. As soon as the platelets are extracted, they are shipped to the alien planet through a cylindrical container, while the human donor is left to die. Dr. Kozmar oversees the operation and is under orders to increase production to satisfy the needs of their population.Faced with a tight deadline, the aliens, who bear a physical resemblance to humans, hire two local gruff and aged auto mechanics, Kurt (Neville Brand) and Fred (Aldo Ray), to help obtain healthy, young donors. Wearing ski masks, the mechanics succeed in abducting unsuspecting college students who loiter at a nearby lake.Nancy (Karrie Emerson) and Ron (Keith Fisher), who plan to marry next month, are kidnapped while camping one night. Their friends, Connie (G.T. Taylor), Heather (Bridget Holloman), and Brian (David Hawk), are awakened by cries, but think the couple is having loud sex and go back to sleep.The next morning, the three friends search for Nancy and Ron around the lake. When they stop in town and inquire at the mechanics' garage, Kurt and Fred claim they saw a young couple hitchhiking to Reno, Nevada. Assuming Nancy and Ron left to elope, the three friends return to the lake.Later that day, Cora stops by the garage to tell the mechanics that she and her associates need ten more test subjects in the next two days for the "research project." She pays the mechanics in gold coins.At the hospital, Nancy protests when Cora tries to give her a shot, insisting she is not sick. Unable to subdue the young woman, Cora sedates her with a ray of green light called "Ultimate Power" that emanates from a special ring. Exercising the ability, however, weakens Cora, and Dr. Kozmar advises her to use the weapon sparingly as it depletes the aliens' power supply.Cora and Dr. Zarma inform Dr. Kozmar that Kurt and Fred often injure the donors during the abduction, making their platelets unusable, but Kozmar tells his colleagues that the two dimwitted mechanics are ideal candidates because they are easily fooled about the experiment and the "gold coins." Kozmar admits, however, that headquarters or the "Network" miscalculated the number of donors in town, not realizing that most of the students would be away for summer vacation.Back at the lake, Connie and Heather sleep in the station wagon to avoid the bugs, leaving Brian at the campsite. In the middle of the night, Fred sneaks into the station wagon and drives it into town as Connie and Heather try to fight him from the back seat. Although both girls make escape attempts, the mechanics succeed in capturing them.Meanwhile, Brian heard the station wagon leaving and flags down a car on the road, hoping to get a ride into town to find Connie and Heather. However, Cora is the driver, and she takes Brian to the mechanics' garage where he is tied up in the basement alongside Connie and Heather. After awhile, Connie slips out of her restraints and attacks Kurt when he attempts to seduce her, but the deranged mechanic recovers and impales Connie with a power drill. Kurt is about to use the power tool to kill Brian when the telephone rings.Meanwhile, at the hospital, Nancy strikes Cora on the head and escapes. Further down the hall, she hides in a room with several dead bodies covered in sheets and is devastated to find her fiance Ron among them. She slips away from the hospital by concealing herself underneath a tarp in the back of Fred's truck.When Kurt returns to the basement, he unties Heather and prepares to rape her, but she uses a wrench to fight back. She flees upstairs, where she and the mechanic wrestle underneath the automotive hydraulic lift. Heather ties Kurt's shoelaces to the base of the lift, and he is crushed as a car is lowered on top of him. Fred arrives and is dumbfounded to see the mangled body of his friend. While he restrains Heather, Nancy emerges from the truck and sneaks up on him. She assaults Fred with an air hose before running away. Wielding an ax, he pursues Nancy into the woods and kills her when she tries to hide inside a shed.Meanwhile, the commuter alien spaceship arrives to pick up the alien team, as Dr. Kozmar assures Dr. Zarma that they can leave without completing the final extraction. They must depart without Cora who was "expunged" after being hit by Nancy.Just as Fred is about to take revenge on Heather for Kurt's death, Brian breaks free from the basement to stop him. While Fred tries to attack Brian with the ax, he is suddenly betrayed by the aliens, as a green ray of "Ultimate Power" from the departing spaceship strikes him down. After Brian unties Heather and embraces her, the two confiscate Fred's truck and escape.