In Washington, D.C., Justice Department agent, Alexandra “Alex” Barnes, is investigating the death of a New Jersey mobster who apparently died from “Ondine’s curse,” a condition by which seemingly healthy middle-aged people die of respiratory failure in their sleep. Alex’s co-worker, Michael, tells her of a New York City publishing magnate, Sam Peterson, who recently died from Ondine’s curse. Peterson, who was sixty years old, had only been married to his thirty-year-old wife, Catharine, for four months at the time of his death. Several months later in Dallas, Texas, Catharine, now going by the name of “Marielle,” is married to rich toy manufacturing mogul, Ben Dumers. Catharine goes to her safety deposit box at a bank and takes out a vial of poison. She fills a syringe with the poison and injects it into the cork of a bottle of champagne. Soon after that, Dumers is dead. After the funeral, Dumers’ sister, Etta, contests the will, saying that Dumers cut her out of it a mere two months after the marriage. Catharine offers her $500,000, which Etta accepts. Back in Washington, Michael tells Alex about Dumers’ death from Ondine’s curse. Alex is Intrigued by the similarities of the Peterson and Dumers deaths, noting that both men had married much younger women a few months before dying, both wives were out of town when the deaths occurred, both wives disappeared shortly after the funeral, and both wives liquidated the assets quickly and transferred the funds to a Swiss bank account. Alex gets photos of the two wives and decides they are likely the same person. Meanwhile, in Seattle, Washington, Catharine, now going by the name of “Margaret,” studies up on ancient artifacts, especially ancient coins. She goes to a local museum, buys her way onto the board of directors, and impresses wealthy museum curator, William Macauley, who begins dating her. He takes her to his large, rustic house, saying he hopes to live there with a family someday. After they make love by firelight, Catharine notices Macauley wears a Medic Alert chain around his neck to announce his allergy to penicillin. Soon after, the two marry. Meanwhile, Alex goes to New York City to interview Sam Peterson’s assistant, Sara, who says she heard from Catharine a few times shortly after she left Manhattan, but nothing since then. Alex, also goes to Dallas to interview Ben Dumers’ sister, Etta, who has not heard from Catherine since receiving the $500,000. Alex returns to Washington where she tells her boss, Bruce, she has narrowed her search to six women. She wants permission to pursue the case further, but Bruce claims she is too obsessed. He suggests she start dating and having a life outside work, but when she insists, Bruce reluctantly approves her going to Seattle.There, she goes to the museum posing as a freelance reporter doing a story on powerful women. Curator Macauley says his wife is very private and will not likely agree to be interviewed. Later, Alex goes to the Seattle police with her suspicions about Catharine, but they dismiss her concerns. Meanwhile, Catharine goes to the doctor complaining of a recurring bout of tonsillitis, and the doctor gives her penicillin. When Macauley tells Catharine about the reporter wanting to interview her, she begins to panic. She grinds the penicillin tablets into a powder and mixes it into a tube of toothpaste. Shortly after that, Macauley dies from a heart attack, but the autopsy does not reveal anything unusual. Alex returns to Washington feeling guilty that she did not warn Macauley about Catharine. She is determined to prove Catharine’s guilt, but her boss has other cases for her to work on. Instead, Alex quits her job and sells her belongings. A clerk at an airline ticket counter in Seattle said a woman fitting Catharine’s description bought a ticket for Hawaii, so that is where Alex goes. There, Alex hires Mr. Shin, a private investigator, but he does not find anything about Catharine. Meanwhile, Catharine, now going by the name “Renni Walker,” is dating rich hotelier, Paul Nuytten, who wants to build a luxury hotel overlooking an active volcano. Alex discovers Catharine taking a SCUBA diving class and introduces herself as “Jessica Bates.” The two are partnered in the class and later go out for drinks. Catharine brags about her new boyfriend and invites Alex to a party Paul is throwing. There, Paul flirts a little with Alex, which seems to please Catharine. A few days later, Mr. Shin approaches Alex at a local farmers market, however Catharine observes them and becomes suspicious. Catharine telephones Sam Peterson’s assistant, Sara, to inquire if anyone has been asking about her. Sara reports that Alex was there several months ago, which upsets Catharine. A few days later while SCUBA diving in the ocean, Alex’s oxygen tank is empty and she panics. However, Catharine comes to her rescue, sharing the respirator from her oxygen tank until they can get to the surface. Later, Catharine says she has had lots of husbands, which is how she got rich. She also encourages Alex to pursue Paul. The next day while Alex, Catharine, and Paul have lunch, Catharine feigns a headache and leaves the two of them alone. Alex and Paul spend the afternoon wandering in the wildness and Paul kisses Alex. Meanwhile, Catharine breaks into Alex’s apartment and searches her belongings. She hires Shin to follow Alex. He easily gets photos of Alex and Paul kissing on her balcony. Catharine pretends to be upset upon learning of Alex and Paul’s affair. Soon after, Catharine and Paul are married. Alex shows up at the wedding reception and accuses Catharine of pushing her toward Paul so he would end up wanting Catharine even more. Alex gives her a black widow spider necklace. Catharine responds by kissing her on the lips. Catharine goes to Shin’s office and holds a gun on him, forcing him to inject drugs into his arm. Shin dies and police find the photos of Paul and Alex while searching his office. A few days later, when Catharine goes to San Francisco, California, Alex tells Paul of her suspicions about his wife. Thinking that Alex is just jealous, Paul reports that his will leaves his entire estate to the Cancer Foundation. By the time Catharine returns from San Francisco, Paul is dead, and the police have arrested Alex after finding a delayed acting poison in her apartment. At the reading of the will, the lawyer reveals that Paul’s legal state of residence was Florida, which has a law allowing any bequest made to a charity to be invalidated if death occurs within six months of changing the will. Catharine indicates she wants all the money for herself. She visits Alex in jail, saying Alex was just an ambitious federal agent who became too obsessed with her. She suggests that when Alex could not catch her, she killed Paul in an attempt to frame her. A moment later, Sam Peterson’s assistant, Sara, comes into the visiting area, followed by a very much alive Paul. Catharine kisses Paul, but he does not respond. Paul and the police faked his death to prove Catharine’s guilt. The police arrest Catharine. All charges against Alex are dropped, and she is released.