Lovely, well-to-do Christine Bravo Penmark has everything: a loving, well-paid husband with a respectable career (as an Air Force colonel, no less), a swank apartment in a respectable part of town, and an adorable, cherubic eight-year-old daughter. But as Col. Kenneth Penmark leaves for an assignment in Washington, DC, the strains that have lurked beneath the surface of the Penmark household now begin to manifest. For example, her daughter Rhoda gives every indication of being a grasping, greedy child, whom their landlady, Monica Breedlove, indulges with extravagant presents that Rhoda gives some indication of not being satisfied with. For another, Rhoda protests loudly and resentfully when reminded that she had lost a penmanship competition, saying that she ought to have won first place, and the medal that goes with that honor.
The apartment-house handyman, Leroy, presents another complication. Though an adult, he seems to have an eight-year-old mind himself. He is also mean and spiteful, and regularly spars with Rhoda.
Rhoda leaves for a school picnic, wearing two shoes that have been modified with iron plates to make them sound like tap shoes. As she leaves, Leroy sprays her shoes with the garden hose, earning a stern reprimand from Monica. Leroy nurses resentment of Mrs. Breedlove, a lustful attitude toward Christine, and a clear enmity toward Rhoda.
At the picnic, Christine tries to sound out Claudia Fern, the headmistress of Rhoda's school, about how Rhoda is fitting in and getting along. Miss Fern at first is effusive in her praise of Rhoda but then becomes evasive and abruptly excuses herself. Christine confesses at this point that Rhoda seems overly mature for her age, in a "disturbing" manner.
That afternoon, Christine entertains her "psychiatry club," at which Monica fairly boasts of having been under analysis by some of the most droppable names in the profession of psychiatry. (That she could actually have been a patient of Sigmund Freud, as she boasts, is a stretch, and her fellow clubbers seem to know it.) Then, as the club starts to discuss a case of a recently convicted serial murderess, Christine finds the conversation disturbing, prompting Monica to tease her about it. The conversation then continues onto the case of one Bessie Denker, a name that Christine recognizes but won't elaborate on.
In the middle of this conversation, someone turns on the radio, and the announcer gives a shocking report: that Claude Daigle, a fellow student of Rhoda's, has drowned on an old, rickety pier that the students had been forbidden to play on or near. As expected, the school bus brings Rhoda home early--and Rhoda, far from being in any to-be-expected state of shock, seems to care nothing about the death of her classmate, and casually asks for a peanut-butter sandwich and a glass of milk, and then goes out roller-skating. The one adult who takes the most notice of Rhoda's casual attitude is the handyman, Leroy. He says flat-out that she's not even sorry that he died, and Rhoda half-innocently, half-sardonically asks why she should feel sorry. As she skates away, Leroy forms a resolve to find something to scare her with, thinking only as an eight-year-old thinks, to "take her down a peg."
A few days later, Claudia Fern comes to visit Christine, and to reveal some startling information: that Rhoda was the last student to see Claude Daigle alive, and that Claude's penmanship medal, which he had worn to the picnic, was now missing. Miss Fern also reveals that Rhoda had been pestering Claude all morning, trying to snatch his medal from him. Furthermore, Miss Fern reveals that a lifeguard had shouted a warning to a girl answering to Rhoda's description, as she was coming off the wharf.
Christine now remembers that Claudia Fern and her sisters never asked Christine to pay for a share of the flowers for Claude Daigle's funeral, and Miss Fern states that she thought that Christine would prefer to send flowers individually. Now Christine asks flatly why Miss Fern would take that attitude, and ask Christine Penmark about the missing medal. In reply, Miss Fern reveals what she had earlier not wanted to discuss with Christine: that Rhoda has no sense of fair play, is a sore loser, and has earned the enmity of all the other pupils in the school. Furthermore, Miss Fern announces that Rhoda will not be welcome when school takes up again in the following fall. This prompts Christine to ask Miss Fern flat-out whether she supposes that Rhoda had anything to do with Claude's death. Shocked, Miss Fern disclaims any such suspicion, but her behavior indicates that, whether she harbored such a suspicion or not, her sisters do.
Their conversation is interrupted when Hortense Daigle, the dead boy's mother, stumbles into the apartment, drunk and barely able to walk in a straight line, followed closely by Henry Daigle, her embarrassed husband. Mrs. Daigle sarcastically observes that Christine is her social superior, an "honor" that Christine hastily disclaims, and then comes to the point of her visit: she demands to know what became of the penmanship medal, and what Miss Fern, concerned as she probably is with liability issues, won't reveal. Mrs. Daigle also reveals that Claude, when found, had bruises on his hand and a crescent-moon-shaped wound on his forehead. In the end Mrs. Daigle becomes terrifically angry, and then almost collapses in tears, allowing her otherwise-ineffectual husband to lead her away. After Hortense leaves, Christine and Miss Fern have a tearful parting. Shortly after this, Col. Penmark calls to ask about the accident, and to reveal that his Washington assignment will last at least another four weeks.
When Monica calls and asks Christine to lend her a locket that she had given Rhoda, so that she can have its gemstone changed, Christine is horrified to find the penmanship medal in Rhoda's little "treasure chest." She then shows the medal to Rhoda and demands that Rhoda explain how she came by the medal. Rhoda at first tries to change the subject, then denies that she had had the medal, then tells a variety of stories that Christine recognizes as false almost at once, and finally says that she had somehow persuaded Claude to let her hold the medal, and then had gone out onto the pier and fallen in later on. In between these activities, Rhoda indulges in a bit of flattery that now seems anything but flattering. Then Christine recalls a fatal accident that had happened to a neighbor in Wichita, KS, an accident by which Rhoda came into possession of a keepsake belonging to the neighbor. Christine finally announces her intention to call Miss Fern, intending to surrender the medal, but Rhoda, in a frightened tone, begs her not to, saying that the school officials don't like her and were persecuting her. Miss Fern is not available, and Rhoda seems to care most of all about retaining the medal, insisting that she, not Claude, had actually earned it.
Back in Washington, Col. Penmark buys an expensive children's tea set for Rhoda and has it shipped to Rhoda, packed in excelsior. Rhoda goes out to play with it, and takes the packing material with her. Leroy sees her at play, and begins his scare campaign by accusing her of beating Claude Daigle with a stick and saying that she could never clean a murder weapon enough to remove all trace of blood, that the police have "stick bloodhounds" that can find a thrown-away stick in any forest, and that the police laboratory will sprinkle "blood powder" on a blood-soaked stick and make it turn blue. Rhoda, for her part, gives Leroy the excelsior, saying that she knows about the bed of excelsior that Leroy has made for himself so that he can sleep on the job in the furnace room without being caught. Nor is she afraid of Leroy's accusations, saying that he made them all up. Christine catches Leroy talking to Rhoda and warns him never to speak to Rhoda again, or she will report him to Monica Breedlove.
When Rhoda comes back into the house, she asks her mother about police tests for blood. Christine offers to ask Miss Fern about it, but again Rhoda does not want Christine to ask Miss Fern any such thing. Christine instructs Rhoda to go up to Mrs. Breedlove's apartment for dinner, while the famous psychiatrist Reginald Tasker dines in the Penmark apartment with the local "psychiatry club." Mr. Tasker reveals that he has great respect for Christine's father, Richard Bravo, who had been quite an authority on crime and criminals in his younger days. Tasker and Bravo engage in good-natured professional sparring, and also discuss Christine Penmark's own dreams of writing a murder mystery. Christine asks Tasker whether children ever commit murder, and Tasker says that they do, and even that many adult criminals begin their careers in childhood. Christine insists that criminals are made in bad environments, but Tasker insists that some criminals are born that way, that they are "bad seeds," possessing atavistic, consciousless minds, and even that one might inherit a tendency to criminality. Richard Bravo refuses to accept such a theory, but Tasker politely stands by his theory, and even states that a child criminal would not present a sour countenance at all, but would instead present a quite convincing "normal" and "innocent" manner.
Then the conversation turns to the case of Bessie Denker, one of Richard Bravo's own case-history subjects. Bravo wants to avoid the subject, but Christine insists, and Tasker reveals that Ms. Denker vanished without a trace before the authorities could make an arrest. Tasker even remembers that Ms. Denker left a child behind, a part of the story that Bravo hastily denies. Shortly after making this revelation, Tasker leaves.
Now that Christine and her father are alone together, Mr. Bravo challenges Christine with the worry that shows plainly on her countenance. Now Christine starts to talk freely of her fear of having been an adopted child, and not the actual daughter of Richard Bravo and his wife. Christine also says that her concerns about Rhoda have prompted a return of her old fear. She mentions a recurring nightmare that she had always talked to Richard about, but Mr. Bravo clearly does not want to discuss it, and especially does not want Christine to lend credence to the inherited-criminality theory.
But now Mr. Bravo reveals that he did find Christine in "a very strange place." Then Christine reveals her recurring nightmare, about living in a farmhouse with her brother and a very beautiful lady, then not having a brother anymore, then being terrified of being in the room, then being outside in an orchard. Finally she remembers the name she originally had: her name was not Christine Bravo at all, but Ingold Denker, Bessie Denker's daughter.
This, then, is the truth: Richard Bravo found Ingold Denker, whom he renamed Christine, in the orchard on the Denker farm after Bessie Denker had died. But Christine now feels that she would have done better to die, because she is afraid that she inherited a tendency to extreme criminality from her real mother, Bessie Denker--and passed this on to her own daughter, Rhoda Penmark.
At this moment, Monica Breedlove returns with Rhoda. Rhoda greets her grandfather cheerfully, but now Richard Bravo takes a long, hard look at his granddaughter, though he will not explain his misgivings. Bravo agrees to stay in a spare room in Monica's apartment overnight.
Later, Christine catches Rhoda trying to dispose of something in a paper bag. Rhoda doesn't want to reveal the contents of the bag, so Christine struggles with Rhoda and forces her to reveal what she had been trying to dispose of: her shoes with the iron half-moon reinforcements. Now Rhoda knows the horrible truth and extracts a confession from her: that Rhoda killed Claude Daigle by beating him with her toe shoes, after he had refused to let her handle his penmanship medal. After the first blow, Claude surrendered the medal, and then tried to escape, and Rhoda struck him again, and again, until he fell in the water. When he tried to climb back onto the wharf, Rhoda beat the backs of his hands with her shoes to make him let go, and so he drowned. Rhoda also confesses something else: that she had in fact deliberately caused their neighbor in Wichita to fall on some ice. Christine, totally distraught, instructs Rhoda to throw the shoes into the incinerator slot and say nothing to anyone. Rhoda again asks about the medal, and Christine half-promises not to give the medal back to Miss Fern.
The next morning, Richard Bravo leaves, and Leroy again tries to tease Rhoda. This time Leroy says that if the police find the stick with which she killed Claude Daigle, they'll sentence her to death in the electric chair, and even that the police have his-and-her child-sized electric chairs. Next, Leroy says that he hasn't seen her in her tap shoes, and now says that he knows that she hit Claude Daigle with the shoes, and even that he has retrieved them from the incinerator. That last boast causes Rhoda to take alarm and demand that Leroy return the shoes. At last Leroy has found something to scare her with, and he presses his point--and finally realizes that she really did the deed, and that's the reason why she is scared and is demanding the return of the shoes.
In fact, Leroy did not retrieve the shoes when he said he did, but after Christine chases Leroy away from Rhoda a second time, Leroy opens the incinerator and finds the shoes, but is now too afraid to say anything. Christine, for her part, reprimands Rhoda for talking about this subject after they had agreed never to speak of it to anyone.
When Monica calls to give Rhoda the locket, after it has been modified, Rhoda asks permission to catch the ice-cream man and buy a popsicle. On her way, Rhoda tries to steal several matches. Christine tells her to replace them, but manages to hide two of them. When Rhoda leaves, Monica probes Christine, knowing that something is troubling her, but not knowing what. Monica offers to give Christine some sleeping pills to help her sleep. Christine collapses in a pool of tears, but still is afraid to tell Monica anything. At that moment, Hortense Daigle, drunk as before, returns to the apartment, seeking yet another confrontation, not only with Christine, but this time also with Rhoda. Hortense has by now tried to talk to Miss Fern a dozen times without result, and has also spoken to the lifeguard from the fatal picnic. What she now knows makes her firmly suspect Rhoda of something, though she is not even sure herself what she suspects. Hortense does not succeed in getting the information she seeks, or in scaring Rhoda (because Monica hastily takes Rhoda away on a pretext). But she does succeed in frightening Christine very badly before her husband finally shows up to take her home again.
After Hortense leaves, Christine starts to call Col. Penmark, but despairs of having anything constructive to tell him. Monica returns and reveals that she allowed Rhoda to go out and buy another popsicle. Rhoda then comes back into the apartment, crosses to the music room, and starts again to play her favorite tune, "Au clair de la lune," on the piano over and over--slowly at first, and then faster and faster until she is playing it at break-neck speed. While she is playing, Christine hears a man's voice from below, screaming at the top of his voice for someone to let him out. She then hears a desperate knocking from inside the sloping cellar door, and then sees a cloud of smoke emanating from that cellar. Monica's brother Emory and Mr. Tasker, who is still staying in the house, try desperately to break the cellar door open, but before they can get in, the screams rise to a blood-curdling crescendo and then stop. The owner of that voice was Leroy, who has, quite simply, burned to death.
Christine now suffers a nervous breakdown and starts to strike the dining-room table with her right hand and babble about being blind, and finally screams at Rhoda to stop playing that piano tune over and over. When Rhoda does come out of the room, Monica has to restrain Christine from striking Rhoda. Christine then collapses in tears as she confronts the realization that Rhoda has killed yet another person. But Christine blames herself, not Rhoda, because Christine is now fully convinced that Rhoda inherited her murderous tendencies from Christine herself.
That night, she induces Rhoda to confess to killing Leroy, which Rhoda does, all the while insisting that the fault was not hers, but Leroy's, because Leroy shouldn't have scared her with his loose talk about police investigations, evidence, and little blue and pink electric chairs for children. Christine reveals at this point that she has dropped Claude Daigle's medal onto the pilings at the pier where he drowned.
Finally, Christine gives Rhoda what she says are "new vitamins," but are actually sleeping pills, in an amount that is obviously an overdose. After putting Rhoda to bed, Christine shoots herself with Col. Penmark's revolver that they keep in the apartment. Two days later, Mr. Bravo and Col. Penmark, both returned from their respective cities, discuss the apparent attempt at murder and suicide. Rhoda recovers completely, but Christine must stay in the hospital to recover. Col. Penmark takes Rhoda home, but Richard Bravo asks the hospital doctor whether Christine had mumbled anything while he was attending to her. The doctor remembers that she kept repeating the phrase "bad seed" over and over. Mr. Bravo says that Christine was about to write a book about inherited criminality, but the doctor, as Mr. Bravo had done, totally rejects the theory, saying that no one would dare bear or adopt children if that were the case.
Col. Penmark puts Rhoda to bed and then waits up. He takes a call from the hospital, as Christine desperately wants to confess to Kenneth that she has "sinned a great sin." The colonel says that whatever the problem is, they will face it together.
Rhoda, meanwhile, has put on her raincoat and sou'wester and gone out of the house. She walks all the way to the Fern school grounds, and onto the wharf where Claude died. Rhoda wants, quite simply, to retrieve the medal. Using a flashlight, she spots the medal underwater, and takes an old fishing net to try to snag it. Lightning strikes her and knocks her into the water.
The movie, as such, ends here--but then a casting-call sequence appears in which the actors playing the various parts introduce themselves. Rhoda appears next to last, and finally Christine--but then Christine steps toward Rhoda, seizes her, turns her over her knee, and gives her a sound spanking. The last frame is a plea to the audience not to reveal the "unusual climax."