Near the end of the Civil War, wounded Yankee soldier John McBurney is close to death when he is rescued by a twelve-year-old girl from a Louisiana boarding school. As John recovers, he begins to charm the girls, becoming the object of their awakening sexuality and jealousy runs rampant. After he rejects the headmistress of the school for a younger girl, John falls down the stairs breaking his leg, which eventually must be amputated. John begins to drink, and his behavior alienates both teachers and students. He makes an attempt to reform, but it is too late, as the headmistress and girls have decided to solve their problem by feeding him poison mushrooms.

In the war-torn South during the Civil War, twelve-year-old Amelia is the youngest of six students at the Farnsworth Seminary for Young Ladies, which is located in a secluded plantation mansion. While searching the woods for mushrooms, she encounters a severely burned and injured Union soldier, Corp. John McBurney. After they introduce themselves by their nicknames, Amy and McB, he playfully kisses her and she helps him to the school. Martha Farnsworth, the headmistress, at first expresses annoyance at having him there. However, she treats his leg injury and puts him to bed, but is careful to lock his room and board up his windows to hold him prisoner. Her slave Hallie watches skeptically, because she remembers the passionate Martha's incestuous relationship with her brother Miles before he left home, a closeness revealed in a portrait of the siblings hanging in the room. Although Martha is required to tie a blue handkerchief on the gate to alert the Confederates stationed nearby that an enemy is present, when the captain rides by, she fails to tell him about McB and justifies her action to the women, saying that he would die in his weakened condition. Stronger than he seems, McB immediately begins charming each of the women in the household, as he realizes that the only thing standing between him and a Confederate prison is their goodwill. His presence in the male-starved household prompts the girls to have thoughts about sexual matters. Although his room is off-limits to the girls, Amy sneaks in, kisses him on the cheek and tells him about her beloved pet turtle, Randolph. When Martha's twenty-two-year-old assistant teacher, Edwina Dabney, a sweet, inexperienced woman, lovingly ministers to his wounds, the oldest student, Carol, a sexually precocious seventeen year old, offers to give him a sponge bath in areas Edwina ignores. Martha, whose latent sexual desire for Edwina simmers unnoticed, tells the younger woman, who is grateful for being given a home, that she is to be her beneficiary. Martha asks Edwina to run the school, so that she is free to manage the money-making farm on the property. Later, McB prods Edwina into admitting that she doesn't trust men and asks her if she is a sleeping beauty waiting for a prince to free her, an idea she finds appealing. Aware that Martha's power is the key to his safety, he also continues to try to beguile her, which requires patience, as the woman is short with him. Too smoothly, he claims falsely that he is a Quaker medic and that he was injured and separated from his troops while saving a "Reb" from a fire. He almost wins her over, but she gets angry when he shows too much interest in the wine she offers for medicinal purposes. Carol sneaks into his room, kisses him, and tells him she knows more than the average girl her age, an admission he has no reason to doubt. Later, while Martha is away on business, the adoring Amy and Hallie retrieve crutches from the barn and allow McB to go outside. When Hallie admits that McB's presence reminds her of the love of her life, a slave who was forced to flee when Miles tried to sell him, McB offers to find him if she will help him escape. To Edwina, McB proclaims a fast-growing love and after he prompts her to claim similar feelings, he kisses her hand. When Martha returns, McB offers to help her on the farm, lying that he came from a family of farmers, and she accepts, although she still plans to lock him up at night. Carol again pursues McB to invite him to her bed. She kisses him, but he restrains himself, cautious about being seen. When Edwina discovers them together, she sends Carol away and, revealing her father's infidelities, demands to know if McB is simultaneously romancing them both. Carol sees McB kiss Edwina when she sneaks back and, jealous, ties a blue handkerchief on the gate. Three Confederate soldiers come to claim McB, but Martha asserts that he is her cousin from Texas. At supper, McB mentions how much he likes the mushrooms. Afterward, Carol lets McB know that she displayed the handkerchief because she disapproved of his attentions toward Edwina. When three lecherous Confederate soldiers come calling and offer to "guard" the girls, Martha politely and firmly talks them into leaving. Later, when alone with McB, Martha serves him wine and asks him to stay at the farm to fill her brother's vacancy. He guesses her implicit desire when she walks him to his bedroom, kisses him passionately and leaves his door unlocked. During the night, Martha has a lurid dream, and McB thinks about the three women available to him. As he knows she expects him, McB walks toward Martha's room, but when intercepted by Carol, follows her resignedly and commences to have sex with her in her attic room. Awakened by Carol's giggling, Edwina goes upstairs and finds them. In the stairwell, she yells at McB, waking everyone, and causes him to fall down the steps. Martha, now aware of McB's alliances with Carol and Edwina, claims that his leg will become gangrenous and insists on amputating it at once. When McB later awakens from drug-induced sleep, he apologizes to Edwina and Martha, but when he realizes his leg is missing, he becomes bitter and abusive, and is again locked in his room. Amy, who tells him about Union troops in the area, is confused, as she thought McB loved her. When Carol apologizes and expresses her continued interest in him, despite having told her housemates that McB forced himself on her, he convinces her to unlock his door. Once free, he shoots Martha's pistol and announces that he will have any girls he wants or he will alert the Union camp. When Hallie suggests he leave, he threatens to rape her, prompting bad memories of Miles's attempt to rape her. After getting drunk in the cellar, he confronts everyone, telling how each woman offered herself, and accuses Martha of cutting off his leg to keep him near her. Amy tries to appease him, but he throws her turtle against the wall, killing it, then feels ashamed. Edwina follows him to his room, blocks the door to assure their privacy, and proclaims her love for him. Meanwhile, Martha decides that McB is a danger to them in or out of the seminary, but vetoes the girls' various violent suggestions for getting rid of him. Instead, she suggests they make him an "excellent supper" and asks Amy, who is angry that he killed her turtle, to gather mushrooms for his favorite dish. Understanding her meaning, Amy says she knows just where to find them. At supper, McB is contrite and blames his behavior on the alcohol. Announcing that he plans to leave in the morning, he promises to arrange for the Union soldiers to offer them the protection extended to "friendly Southerners," and Edwina says that she is leaving to marry him. Unaware that no one else is eating the mushrooms, McB compliments the dish, and serves Edwina a helping. When she starts to take a bite, Martha yells out to stop her. McB rises, his head spinning, and leaves the room, followed by Edwina. The others hear a crash, but finish their meals. Later, the girls are taught how to sew a shroud. When they discuss how the mushrooms killed McB, Martha denies it, explaining that, because of all he had been through, his heart "gave out." For the last time, Edwina tells McB that she loves him, and the women carry his shroud-covered body out the gate to be buried.