The Story of an Hour by Kate Chopin is, literally, that; one hour in the life of a woman. The hour changes her life completely but not in the way the reader or Louise Mallard, expects. The reader anticipates the trauma and depression that should accompany the news of the death of Louise's husband. Louise Mallard however, comes to a stark realization that she will be "free!" She is a typical wife of her time; dutiful and obedient but not unhappy. However, she has no real capacity to make decisions or to be her own person. Now she can see a future for herself and, "she would live for herself. There would be no powerful will bending hers in that blind persistence."
Mrs Mallard "breathed a quick prayer that life might be long. It was only yesterday she had thought with a shudder that life might be long." The irony of these words will shortly be intensely felt by the reader. Louise's feelings do not last long as her husband, presumed dead, returns home, having been nowhere near the site of the accident which was presumed to have killed him. In that instant, he comes to represent the conflict in the story because this conflict, the thing that has caused Louise's unexpected reaction and subsequent relief, has re-entered her life. She is, once again, subject to society's expectations and restrictions; something that, as a widow, she could have been freed from.
This allows the reader to expand his or her view of the antagonist to include more than one possibility. Even Louise Mallard's own character is something of the antagonist as she has been submissive all these years and has chosen this role herself, rather than ever taking control of her life. Her decision to only now look forward to the future seems heartless and uncaring.
The reader, then, can consider the antagonist as being the expectations of society and marriage and the conflict it creates, and also Louise's own role and even the role of her husband in perpetuating the biased and intolerant situation that has developed; a situation apparently, so untenable that, when Brently Mallard returns, the shock actually kills Louise.