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Wealthy New York invalid Leona Cotterell Stevenson's lifeline is the telephone.  Consequently, when she is left alone after her maid departs and gets a continuous busy signal from her husband Henry's office, she becomes worried and asks a telephone operator to connect her.  Leona is accidentally connected to another call, during which she hears two men planning to murder a woman at 11:15 that night, while a train passes on a bridge.  Horrified, Leona asks to trace the call, but the operator refuses, and the police are uninterested in her vague information.

Leona then learns from Henry's secretary that he left the office in the afternoon after making a lunch date with a blonde named Sally Lord.  Leona calls Sally, who does not want to speak in front of her husband, but agrees to call Leona back from a phone booth.  Before the call goes through, Leona recalls Sally, who has told Leona that her maiden name was Hunt: Sally is dancing with Henry one night at a college dance when Leona, the spoiled daughter of a pharmaceutical manufacturer, boldly cuts in on them.

Leona hotly pursues Henry, a high school dropout, who is defensive about his lower class heritage.  Indifferent to Sally's heartfelt confession that she loves Henry, Leona decides to marry him.  Her domineering father, James Cotterell, protests the marriage but gives in after Leona becomes hysterical, and soon hires Henry to work at his company.

Not long after marrying, Henry realizes that Leona intends to use her money to control him.  Leona's recollection of her former rival brings a sneer to her face, and after she picks up Sally's call, she learns that Sally's husband Fred, who works for the district attorney, is investigating Henry.  Sally tells Leona that out of curiosity, she followed Fred from work one day: Fred and several investigators go to make a mysterious pay-off in an abandoned house on Staten Island.  Sally later tries to warn Henry during their lunch date, but he is distracted and soon disappears.  After hanging up with Leona, Sally follows Fred to a subway station, where she calls Leona again with news that the house on Staten Island burned down, three men have been arrested, and the situation is somehow tied to the Cotterell company.

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Leona next receives a telegram message by phone, informing her that Henry will be gone for the weekend.  Hearing a train cross a nearby bridge, Leona suddenly fears for her life.  Distraught, she calls her physician, Dr. Alexander, away from his dinner.  The doctor reveals that ten days earlier, Henry sought a consultation with him about Leona's condition, which prompted Henry to recall the first time he learned of Leona's illness.  Six months after marrying, Henry breaks a lunch date with Leona to meet with a prospective employer because he is frustrated at Cotterell.  Leona demands that Henry continue to work for her father and when he refuses, they argue and she later collapses from a heart attack.  Henry becomes further embittered after James uses his influence to prevent him from getting work elsewhere, and Leona continues to humiliate him.  Leona's attacks persist until she becomes an invalid.

Although Dr. Alexander diagnoses Leona's problems as psychological, Henry asks him to wait before informing her.  The doctor tells Leona that he has not heard from Henry since.  Leona's next call is from Waldo Evans, a chemist at Cotterell, who, after giving her specific messages for Henry, recounts how he became involved with him.  The now embittered Henry convinces Evans, who dreams of buying a retirement home in England, to falsify Cotterell chemical reports and sell portions of the pharmaceuticals to a fence named Morano.

After seven successful months, Evans is transferred to the company's New Jersey plant, and Henry and Evans go into business for themselves, selling the goods out of the house on Staten Island.  Angered by their betrayal, Morano later confronts Henry and Evans and demands $200,000 and their remaining stock of drugs in exchange for their lives.  When Henry protests that he has no money, Morano suggests that Henry pay him with Leona's life insurance money, as he knows that a Chicago doctor has given Leona only ninety days to live.  Backed into a corner, Henry agrees to the deal, but when Leona lives, as he knew she would, Morano refuses to give him more time.

Leona finds out that Morano has since been arrested, and that Evans burned the evidence in the Staten Island house.  At 11:00, a man breaks into Leona's kitchen downstairs, just as Henry is calling her from New Haven.  Henry initially denies his criminal activity, but when Leona reveals that Morano has been arrested, Henry panics.  In tears, Leona sobs to Henry her apology for abusing him, and Henry, who has taken out a contract on her life, desperately urges her to go to the window and scream for help.  Leona is paralyzed by fear, however, and hangs up the phone just before she is murdered.  Henry, who is about to be arrested, calls Leona back.  The murderer picks up the phone, and when Henry asks for his wife, responds, "Sorry, wrong number."