Marion Ravinel is married to Dr. Frank Ravinel, an arrogant abusive doctor. Together they run a physical rehab spa at what was once Marion's family home in New Orleans. But a congenital heart condition forces Marion to curtail her activities. The condition is controlled with medication but she must still use a wheelchair at times. Frank is brusque and rude to the staff, especially Laura Morrell, a physical therapist. After witnessing Frank strike Marion, Laura befriends her. Both women are afraid of Frank, although Marion professes to still love him and keeps hoping he will change.
Among the spa employees is Evangeline, an elderly African-American woman who has known Marion all her life. Frank has tried without success to evict Evangeline from her apartment on the premises. He considers her a bad influence on the clients because of her belief in voodoo. But she insists that Marion's parents promised her a home for life.
After learning that Frank is planning to sell the property behind her back, Marion confronts him. He is physically abusive and Laura comes to her aid. Frank threatens both women. It turns out that he has a hold on Laura. She served time for killing a man who tried to rape her and is actually on parole. If she violates her parole, she will have to return to prison. Frank is aware of this and frequently threatens to turn her in on trumped-up charges. Although Marion is somewhat taken aback to learn that her new friend is a murderer, she eventually agrees that it was justified.
Evangeline is afraid for Marion and warns her to be careful. Laura convinces Marion that neither of them are safe. She proposes murdering Frank and disposing of his body. Marion is horrified but when she is once again subjected to her husband's physical abuse, she agrees to Laura's plan.
The women drive to a house on the bayou owned by Laura. The plan is to lure Frank to the house, knock him out with drugs, and drown him in the bathtub. It is Mardi Gras and everyone is celebrating. The neighbors, who are acquainted with Laura, are having a noisy party. No one will notice Frank's arrival. Marion gets cold feet at the last minute but Laura tells her it is the only way they can ever be free of Frank. Reluctantly Marion agrees. She telephones Frank and asks him to come to the house so they can talk things over. To her surprise, Frank knows the house belongs to Laura and he appears to have visited there before.
Laura tearfully confesses to having an affair with Frank. She insists he forced her into it and threatened her. By this time Marion is not sure what to believe. She no longer trusts Laura but feels it is too late to back out. When Frank arrives and is served lemonade containing narcotics, Marion knocks the glass from his hand. She cannot bring herself to kill the man she once loved. Frank taunts her and pours himself another glass. This time she lets him drink it. He falls asleep. With difficulty, Laura and Marion drag him to the bathtub full of water and put him in. He appears to drown. Laura orders Marion to lie down and rest, fearing she will have a spell with her heart.
The next morning, they roll Frank's body in a canvas and drag it to Laura's car. After they have him inside and the door closed, Laura's friend from next door appears. He is disappointed that they are leaving. Then he notices water pooling next to the car door. When he offers to look inside and see what is leaking, Marion curtly orders him off.
The women drive both cars to a lake. Frank's car is pushed in and they watch it sink. Then they return to the spa. It is early and no one is about. They drag Frank's body to the man-made pond and roll it in. Evangeline sees them but says nothing.
Soon everyone is looking for Frank. It is unlike him to miss appointments with the clients. While watching a VHS tape of Mardi Gras, Marion spots Frank in the crowd, wearing the costume that was specially made for him. She faints and is confined to bed. Another doctor at the spa warns her that her blood pressure is sky-high and she should be in a hospital but she refuses. She is disgusted with Laura and wishes they had not done it.
The next morning Frank's car is brought home by his uncle, who saw him the day before. Laura and Marion are shocked but manage to act normal. Marion learns from the uncle that Frank is staying in town at a hotel. She goes there and apparently just misses him. His cigar is still smoking in the ashtray. But how can any of this be real since she saw Frank's corpse? Laura knocks the handyman's tools into the pond, forcing him to drain it. There is no sign of Frank. This brings on another heart spell for Marion.
Then she hears a report on TV of an unidentified body of a man turning up. Summoning what little strength she has, Marion goes to the police station and tells them she thinks the man is her husband. Sgt. DuBois takes her to view the body but it is not Frank. Sensing she has found a friend, Marion confesses everything to DuBois. He is skeptical but agrees to pay a call at the spa.
After examining Frank's car, DuBois finds traces of mud under the hood. The canvas that Frank was wrapped in is discovered in a cistern. He visits Marion, who is still confined to bed, and tells her he thinks Frank is alive and pulling a fast one on her. This would mean that Laura was also in on it. He promises to get at the truth.
Marion is awakened by someone on the lawn outside. It is Frank, dressed in his Mardi Gras costume. He beckons Marion to follow him. She does and he leads her down the road to a cemetery. Then he disappears in the crowd. Breathing heavily, Marion manages to return home. She goes to the bathroom for her heart pills and sees Frank's body in the bathtub, just as it was that night on the bayou. He rises from the water just as Marion collapses. Her pills scatter everywhere and Frank's shoes crunch them as he walks across the room. He checks Marion's pulse but there is none. She has died of a heart attack. Frank goes into the bedroom where Laura is waiting. He tells her Marion is dead and they fall into each other's arms.
At Marion's funeral, DuBois confronts Frank and Laura. He has deduced that Frank murdered Marion to get his hands on her property. But he cannot prove it and Frank knows this. DuBois promises to be their worst nightmare, which they find amusing.
That night, Frank and Laura serve an eviction notice on Evangeline. She casts a spell and talks to Marion. She apologizes for not being able to save Marion but Frank and Laura were too strong for her. Then she calls Marion to come forth and take her revenge.
Laura and Frank are asleep when he is yanked from the bed by Marion, who appears to be very much alive. She slaps him around the room until he begs for mercy. Then he wakes up. It is morning and Laura is in the shower. Frank smiles with relief until he finds Laura dead in the bathroom. She has been strangled with a pendant belonging to Marion. An expression of horror is on her face.
The police are called and Frank is arrested for the murder of Laura. He insists that Marion came back and killed her but no one listens. Evangeline looks up and sees Marion standing in the bedroom window, smiling and happy. DuBois sees her too but when he looks again, she is gone.