John Rock, a human spider whose comfortable income allows him to ride about in a motor car to look for victims, foolish girls who are attracted by his promises of pleasure, while driving about has trouble with his car. His motor stops before the home of Joan, a whole-souled girl, living with her younger sister, Millie. Joan at once recognizes Rock, having seen his picture in the paper. She tells Millie how Rock snares innocent girls and ruins them. Left alone, Millie flirts and innocently accepts an invitation to go riding with him. That night Joan finds Millie gone and as the hours pass and she does not return, Joan connects her absence with the visionary face of John Rock. She goes to his home, where Rock explains that Millie had met her death in an accident. Realizing that Millie like the other innocent girls had been a victim of the "Spider," Joan attacks Rock with all her strength. The latter calls the police and later, on a trumped up charge, has Joan sent up the river for ten years. Two years of prison fare and misery hasten a deadly desire for vengeance in Joan's heart and one day Joan was freed. As if guided by an unseen hand she visits a notion shop and then calls on John Rock, who has become a nervous wreck through drink and dissipation. Rock is so horrified at seeing Joan and the knife that he falls over dead. Then Joan went forth into the twilight knowing that Justice had righted the wrong done her sister and herself.