The plot of Kate Chopin's "The Story of an Hour" is much simpler than the theme or meaning, but both rely heavily on the use of dramatic irony--a contrast between what we expect to happen and what actually happens.
Louise Mallard is "afflicted with a heart trouble," so when a family friend and her sister learn that Louise's husband has died in a train accident,
great care was taken to break to her as gently as possible the news of her husband's death.
When she first hears the news, she lets out one scream, but then she goes upstairs alone to her room. The bulk of the story is what Louise thinks about when she is alone with her thoughts (more later); her musings are finally interrupted by her sister, who is worried about Louise being alone. Louise lets her sister help her walk back down the stairs; as they reach the bottom, the door opens and Brently Mallard, her husband, walks into the front door. Louise has a weak heart, remember, and she promptly dies of a heart attack. That's really the entire plot of this story which takes place in the space of an hour.
The real action of the story all takes place in Louise's mind as she contemplates life without her husband. Though she "had loved him--sometimes" and would miss some things about him, her overwhelming feeling is freedom.
There would be no one to live for during those coming years; she would live for herself. There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature. A kind intention or a cruel intention made the act seem no less a crime as she looked upon it in that brief moment of illumination.
She sees that her future, which just yesterday she had thought would drag on forever, will now be free forever. She can make her own decisions, choose the things she does and does not want to do, and will never have to be subject to the whims or the will of her husband. Well meaning or not, her husband controlled her life, and she is now free.
The first irony of the story is that what should have been grief is first relief and then exultation. The second and greater irony is that her heart was perfectly capable of handling the tragic news of her husband's death, but it failed her when she sees that her dreams of freedom from the last hour will not come true. That is the "joy that kills," not the joy of learning that her husband is alive, not dead.