Police Lt. Will Jethroe goes to see businessman Tim Hathaway at his home. The clearly drunk Tim lets him in and Jethroe explains that someone shot a woman at the Clover Club... and the shooter's description matches Tim. Tim finally says that the dead woman is his wife, Joanne. Joanne was an angel... and Tim had to kill her.
Five years ago, Tim and Joanne marry and are soon living comfortably in Tim's apartment. One night they prepare for a dinner with their friend, Dr. Jack Farleigh. As Tim helps Joanne with her dress, he asks her if her injury from her recent fall two days ago is still bothering her. She assures her husband that it's nothing even though Jack took x-rays and said that she had a broken rib. Joanne says that she's a fast healer and Jack probably made a mistake.
Jack arrives and Tim goes to get his friend a drink. Joanne approaches the doctor, who tells her that the initial x-ray two days ago confirmed that she had a broken rib. However, the x-rays he took earlier that day show no sign of injury. Jack insists that it isn't possible for someone to heal that fast in two days, but Joanne suggests that he checked the wrong set of plates the first time.
Later after dinner, Joanne turns in for the night. Once they're alone, Jack shows Tim both sets of x-ray plates and insists that they're identical... except that there's no sign of a broken rib in the second set. He tells Tim that there are other medical anomalies since he examined Joanne a year ago. Her heart is now half of its original and her appendix has disappeared. Tim confirms that Joanne hasn't had injury and there's no scar, and Jack insists that there is something seriously wrong with Joanne.
Once Jack leaves, Tim goes in to see Joanne. She's seated at her dressing table and asks what the two men talked about. Tim confirms that Jack described Joanne's strange medical conditions, and Joanne finally tells her husband that she's changing. Not only is her body changing, but she's developing mental abilities as well. Concentrating, Joanne uses the power of her mind to knock over a bust, and explains that she's becoming a different type of human being, more evolved. Tim figures that she's suffering from some kind of delusion, but Joanne shows him a series of radiology textbooks and explains that she's reading and absorbing the knowledge, and her intellect is developing at an incredible rate.
Shocked, Tim wonders about her feelings for him and Joanne tells him that she's already evolved far beyond him. She can't change even if she wanted to and warns that her love for him will soon become the kind of love a human has for a pet or another lesser creature. Joanne insists that they should separate now while they still have some pleasant memories.
Tim tells Will that Joanne disappeared the next morning with no trace of where she went. There was no one who would believe his story, and nothing he could do. However, one day a year later, Tim saw a photo of Joanne in the newspapers. She was using a different name and working in a laboratory at Berkeley, CA. According to the story, she was developing a new electronic-radiation process.
Tim flies to Berkeley and confronts Joanne in her lab. He begs her to come back, figuring that she rigged the statue and the x-rays so she'd have an excuse to leave. Joanne talks to him privately and says that she has continued changing since last they met, and she is the first of a new race of beings. She tells Tim to stay there and she'll get her coat so they can talk somewhere privately. However, Tim discovers that the room she's entered is a broom closet and that she has disappeared without a trace.
Will has a drink, shocked by Tim's story, and Tim explains that he continued reading newspaper articles about Joanne, using different names, making new discoveries in radiation across the world. Four years passed until that day, and Tm finally saw her again. He had bought a gun and was contemplating suicide, and went to the Clover Club for a drink.
Tim enters the Clover Club and sees Joanne sampling drinks at the bar. He goes to her, desperate, but Joanne tells him that she's waiting for someone and wants nothing to do with him now that she is a different species. Joanne boasts that the time is coming for her people's rule over the planet sooner than she realized. When she realizes that the bartender is listening to them, she kills him with her powers and Tim realizes that she's no longer human. Declaring that she's a cold, ruthless killer and no one is safe, he takes out his gun and shoots her, and then runs out of the club.
Will has Tim confirm that he came right back to his apartment after shooting Joanne. Tim says that it was an execution and that now they have to seek out the other humans that have evolved into the new beings. Will tells Tim that Joanne is still alive thanks to her powers, and that she was at the bar waiting for Will. Tim realizes that Will is another evolved being like Joanne and tries to get to his gun, but Will kills him with his powers and says that they must keep their existence a secret… for a little while longer.