A young model, Valerie, is burned in a tanning salon accident. Five years later, Valerie's twin sister Rhonda Johnson, Marcia Karr, is operating a fitness club in Los Angeles. Rhonda chastises one of her employees, Jaimy, Teresa Van der Woude, for missing an aerobics class she was supposed to lead. Shortly after, a killer brandishing a large safety pin enters the women's locker room and slashes Rachael, Teresa Truesdale, a client, to death. While closing the club, Jaimy finds her corpse stuffed in a locker, as well as a blood-soaked knitting needle in the locker of Diane Matthews, Laurel Mock. Detective Morgan, David James Campbell, is assigned to the murder case and questions Rhonda and Jaimy.
The next day, Chuck Dawson, Ted Prior, is unexpectedly hired by Rhonda's senior managing partner of the club. Chuck gets into a physical fight with patron Jimmy Hallik, Fritz Matthews, which impresses club patron Debbie, Jimmy's ex-girlfriend. Later, Morgan learns that Diane's knitting needle was not the weapon used to kill Rachael. Later that night, a group of teenagers vandalize the club with spray paint, only to be brutally murdered by an unseen assailant.
The next day, Tom, Richard Bravo, a club patron whom Morgan has deemed a suspect, is murdered in the locker room, and later, Jaimy is found hanged inside the club. Rhonda harangues Morgan for her perceived ineffectiveness of his investigation. Morgan uncovers that Rhonda is in fact Valerie, who survived the tanning accident, and that she never had a twin sister—instead, she legally changed her name to Rhonda and has kept her badly-scarred body hidden with clothing and a wig.
Morgan confronts Rhonda with his discovery, and she admits it, but remains stern and impervious to his assertion that she may be the murderer. Shortly after, Chuck is murdered. Morgan tracks Jimmy to a nearby concrete plant, believing that Jimmy has taken the fall as a suspect given there is no evidence linking him to the crimes. Morgan confronts Jimmy at gunpoint, questioning him if Rhonda committed the murders, but Jimmy manages to incapacitate Morgan. Jimmy returns to the gym and is shot to death by Rhonda, but not before admitting he killed Chuck out of his love for her, hoping to implicate himself to spare her from being suspected by police.
Rhonda claims self-defense, and Jimmy is widely believed to have committed the serial killings. Some time later, Rhonda is brought to a wooded area by Morgan. When asked why they are there, Morgan recounts a story of his police officer father, who enacted vigilante justice against a serial killer whose case was dismissed on a technicality. Knowing that Rhonda will not be prosecuted for her crimes due to Jimmy's involvement, Morgan attempts to kill Rhonda, but she manages to kill him using a shovel.
Rhonda returns to her gym to oversee it, and is congratulated by her patrons, who herald her as a hero. In her office, she removes the large safety pin from her desk and fondles it, smiling.
