"Say It Isn't So" is a romantic comedy that follows the misadventures of a young man named Chris, played by Chris Klein. After a whirlwind romance with a beautiful woman named Heather, portrayed by Heather Graham, Chris discovers that she is actually the sister of his estranged father. This shocking revelation leads to a series of comedic and awkward situations as Chris navigates his feelings for Heather while trying to keep the truth from her. As the story unfolds, misunderstandings and outrageous scenarios ensue, ultimately forcing Chris to confront his family's past and his own desires. With a blend of humor and heart, the film explores themes of love, identity, and the complexities of family relationships.