The private investigator Mark Higbie is drinking in a bar with his friend Frank, who is waiting for his fiancée Leigh Lockridge, and the bartender Carlo Lenzi. Out of the blue, a young woman asks Frank to help her to go to her car, but something happens and Frank screams, and his friends find his severed body. Soon Leigh goes to Mark's office and hires him to find what happened to her fiancé since Lt. Gully Conti believes she is involved in the murder. Her last husband vanished in a ship during a voyage with her and her twin sister Laura. Mark visits Gully in the precinct and learns that the suspect is a woman named Valerie Steffan. His further investigation shows that there were three previous cases similar to Frank's one, and all the bodies had their fluids sucked out of them. Mark's assistant researches the Lockridge family and learns that their parents died in a plane crash and an old Indian found the sisters, but one of them almost died bitten by spiders. Further, in accordance with ancient legends, women affected by a curse turn into giant spiders during the full moon and under stress, killing their victims. Mark proceeds his investigations and starts to believe that the legend might be true. Now he wants to know whether Leigh or Laura was the child bitten by spiders. And who is Valerie Steffan?