Rent Cactus Flower (1969)

Winston (Walter Matthau) is stringing along his dizzy blonde mistress Toni (Goldie Hawn), by telling her that he has a wife and children. But when he learns that she has tried to commit suicide over him, he promises to marry her. Toni, refusing to be a home wrecker, insists on first meeting Winston's wife, so he convinces Stephanie (Ingrid Bergman) - his starched and no-nonsense nurse - to pose as his wife. Winston's scheme leads to unforeseen twists and surprises for everyone.