Wealthy invalid Leona Stevenson finds herself completely alone in her New York City townhouse. Thinking that her husband Henry would be home, she gave her nurse and the servants the night off. But Henry never came home from work and Leona is frantically trying to locate him. His office number is busy. In desperation, Leona telephones her husband's secretary, Miss Jennings, at home. She learns that Henry left the office with a Mrs. Lord and did not return.
In flashbacks, it is revealed that Leona is the spoiled only child of J.B. Cotterell, the owner of a pharmaceutical company based in Chicago. Her mother died in childbirth and Leona grew up believing that she had a heart ailment. In college she met Henry Stevenson, who came from a dirt-poor background. Although he is dating her best friend Sally, Leona makes a play for him and succeeds in taking him away from Sally.
Against the wishes of her father, Leona marries Henry and he goes to work for her father. But he is unsuccessful in persuading Leona that they need a home of their own instead of living with Mr. Cotterell. Leona has a fainting spell and requires a nurse to take care of her. This has happened many times over the years and Mr. Cotterell blames Henry for this latest episode. Henry tries to find other employment but Mr. Cotterell has put the word out not to hire him.
Returning to the present time, Leona takes a call from a Mr. Waldo Evans. He has called several times wanting to speak to Henry but won't say what he wants. In telephoning Henry's office again, Leona is accidentally connected to a conversation in which two men are talking. They cannot hear her but she hears them plotting a murder. A woman is to be killed in her home at 11:15 that night. An overheard train will cross the tracks and the cop on patrol always takes a break at that time. No one else will be in the house and their client has arranged to leave a downstairs window unlocked. Their client wants them to make it look like a robbery and he doesn't want the woman to suffer.
Leona is horrified and calls the operator, demanding that the call be traced. But the operator doesn't understand what she means about being cut in on a conversation and refers her to the police. The officer who takes Leona's call doesn't believe her. In frustration, Leona hangs up but continues to worry about the unfortunate woman who is set to die.
In looking through the telephone messages that came in earlier that day, Leona finds one from the mysterious Mrs. Lord. She telephones the woman and demands to know what is going on with her and Henry. To her surprise, Mrs. Lord turns out to be her old friend Sally. By now Leona is thoroughly bewildered. Sally explains in a low voice that her husband works for the District Attorney and that Henry is in trouble. She came to his office to warn him. Then she hangs up abruptly after telling Leona that she will call back when she can speak privately, as her husband is home.
Leona cannot imagine what sort of trouble Henry could be in. She telephones her father in Chicago but he is having a party and doesn't give her a chance to explain why she is calling. He is surprised and angry that she is in the house alone and promises to have a talk with Henry.
The phone rings again and it is Western Union with a telegram from Henry. He is enroute to a pharmaceutical convention and will telephone her later. Then Sally calls from a pay phone, having told her husband that she was going out for beer. She doesn't know exactly what trouble Henry is in, only that it is very serious. Earlier that week she followed her husband and one of his colleagues to a deserted house on Staten Island. It appeared to be a drop-off point and Sally heard Henry's name mentioned. Sally is forced to hang up because the drugstore is closing, leaving Leona frustrated and upset.
Mr. Evans calls again and this time Leona demands to know why he keeps bothering her. She is a bedridden invalid and cannot take this kind of stress. Mr. Evans reveals that he knows all about her, having been a Cotterell employee for many years. He is in charge of a laboratory; first in Chicago and now in New Jersey. One night when he was working late, Henry stopped by. He looked around the laboratory and asked where the narcotics were kept. Mr. Evans showed him but felt uneasy about it, even though Henry was the boss's son-in-law. Later Henry offered Mr. Evans a ride home as it was raining and his bus was late. During the ride, Mr. Evans shared his desire to retire and return to England. He would like to purchase a horse farm but doubts he will be able to afford it. Henry agrees and offers to cut Mr. Evans in on a deal. If he will hold back some of the narcotics, Henry has a purchaser. They will each receive a large cut, which will go a long way toward purchasing the horse farm. Mr. Evans is horrified and refuses, but greed overtakes him and eventually he agrees.
All goes well until Mr. Evans receives a sudden transfer to the New Jersey plant. Henry assures him that it is routine, as he would have been arrested if they were found out. Mr. Evans declares that he wants out but Henry won't allow it. He has already decided to act on his own instead of going through their purchaser, a Mr. Morano. Their share of the money will be a lot more. Henry and Leona move to New York, where Mr. Evans purchased the deserted house on Staten Island. He takes the drugs there and someone in a boat picks them up.
Unfortuately, Mr. Morano traces them there and is waiting in the house when Mr. Evans arrives. He wants $150,000 as compensation for the business they took away from him. If they don't pay, they will be killed. When Henry protests that he doesn't have that kind of money, Mr. Morano reminds him that he has a rich wife. An accident can be arranged and he would inherit her money. Henry is sickened but realizes he has no choice. Mr. Evans closes the conversation by giving Leona a message for Henry. The game is up as the police have arrested Mr. Morano. Mr. Evans has burned down the Staten Island house. After midnight he can be reached at a certain number, which Leona writes down. It turns out to be number of the city morgue. Mr. Evans obviously intends to commit suicide rather than be arrested.
Leona telephones her father again but there is no answer. She begins to realize that the phone conversation she overheard was about her. She is the woman who is to be killed that night. Believing that her heart is about to give out, she telephones her doctor and begs him to come over. He can't, although he tells her that the results of a recent examination were very good. She does not have heart trouble at all. The doctor sent her a letter with this information but Leona never got it.
She calls a nurse's registry but they don't have anyone they can sent over at a moment's notice. Hysterically, Leona tries to get out of bed but she can't make her legs move. Then Henry telephones from the train station in Connecticut and Leona tells him she knows everything. She asks why he didn't come to her for the money, that she would gladly have given it to him because she loves him so much. Then she hears a noise downstairs and screams at Henry that someone is in the house. He orders her to get out of bed and yell out the window for help. But Leona is paralyzed by terror. He admits to having arranged her murder and tells her unless she does as he says, he will burn. The two plainclothes detectives who have been following Henry close in on the phone booth. Then Henry hears Leona's dying scream as she is stabbed. The killer picks up the phone, says "Sorry wrong number" and hangs up.