Mrs. Mallard is given the news that her husband has been killed. She retires to her room, and people assume she is overcome with grief. In reality, she is experiencing the relief of freedom, from what is obviously a lackluster marriage. She is rejoicing in her new-found freedom.
The ironic twist comes when she learns her husband was not killed , after all, and then the grief comes into play. She mourns her fleeting moment of freedom instead of rejoicing her good fortune with her husband's return.