Plot
Justice Department agent Alexandra Barnes grows suspicious of the deaths by respiratory failure of several seemingly healthy wealthy men due to a rare condition called Ondine's curse, each married within months to younger wives. Comparing photographs, Alexandra determines the wives are the same woman, wearing different hair colors and styling to look older as a disguise.
One victim is Manhattan publishing magnate Sam Peterson, whose younger wife of six months, “Catharine” (her true name is never disclosed), inherits his estate. Next, Catharine relocates to Dallas, Texas, affecting a southern accent and posing as "Marielle". Seducing and marrying toy company owner Ben Dumers, she subsequently poisons a bottle of expensive liquor, which kills him while “Marielle” is on a trip away from home. After Ben's death, his sister, Etta, unsuccessfully attempts to contest his will, but is silenced by Catharine's gift of $500,000.
After extensive research, presenting herself as an anthropologist named “Margaret,” Catharine buys her way onto the board of directors of a local Seattle museum. “Margaret” dazzles wealthy curator William McCrory, who has a convenient allergy to penicillin, resulting in a whirlwind romance and marriage.
Tracking Catharine’s victims, Alexandra interviews their families and associates, ultimately tracking her to Seattle. Presenting her research to her skeptical superior, Bruce, Alexandra persuades him to send her to Seattle to investigate. Posing as a freelance reporter writing a story on powerful women, Alexandra interviews William, who tells her that his wife is private and will likely decline an interview. Meanwhile, Catharine obtains a prescription for penicillin, which she mixes into William's toothpaste, triggering a fatal heart attack. William's autopsy shows nothing unusual. Alexandra is dismayed but trails Catharine, who she has not yet met, to Hawaii.
In Hawaii, Catharine (now “Renni Walker”) seduces French hotelier Paul Nuytten. Posing as "Jessica Bates," Alexandra enrolls in a scuba diving class Catharine is taking, and the two partner during lessons and become friendly. Catharine grows suspicious on observing a meeting between Alexandra and a private detective, Mr. Shin. Contacting family and associates of previous victims in her pose as “widow,” Catharine learns of Alexandra’s interviews several months prior.
During a diving excursion several days later, Catharine saves Alexandra when her scuba gear malfunctions. Catharine confides in Alexandra that she amassed her wealth from marrying rich men and encourages Alexandra to pursue Paul. While Alexandra and Paul spend an afternoon alone, Catharine breaks into Alexandra's apartment and hires Shin to stalk her. Shin takes photos of Alexandra and Paul kissing. Though Catharine pretends to be upset, a short time later, Paul and Catharine are married.
At “Renni” and Paul’s wedding, Alexandra accuses Catharine of manipulating her and gifts Catharine a black widow spider brooch. Catharine responds by quickly kissing Alexandra on the lips and abruptly departing. Later, Catharine visits Shin, forcing him at gunpoint to administer himself a lethal overdose of heroin. Police find the photos of Paul and Alexandra in Shin's office. When Catharine goes on a trip to San Francisco—her modus operandi when poisoning— Alexandra warns Paul, revealing her investigation against Catharine. A skeptical Paul informs Alexandra that “Renni,” who is independently wealthy, insisted they both amend their wills leaving their entire estates to a cancer charity. At the subsequent notice of Paul’s death, police arrest Alexandra, having found poison that Catharine had planted in Alexandra's apartment, and attributing to her a motive of jealousy on being spurned.
Under Florida law, Paul’s bequest to charity can be invalidated by his wife within six months. A triumphant Catharine visits Alexandra in jail. Moments later, Paul appears, having faked his death to entrap Catharine. Believing the poison may have not been detected, Catharine warns Alexandra that she injected poison in the liquor bottle, revealing her concern that Alexandra not die accidentally. Alexandra informs her that they found the poison, and Catharine is arrested.
