Door Mat Heroine, Appetizer of Revenge

A heroine who had been mistreated by her stepmother and half-sister. Her husband, who married her for political reasons, keeps his mistress in the main residence and treats his legitimate wife, the heroine, with contempt. She is ordered to raise the mistress's child as her own biological child, and she plans her revenge. Three months after the child's birth, both families gather for a dinner as a "coming-out celebration for the child." What will be revealed there—?