In "Wedding Night" by Sophie Kinsella, the story revolves around two sisters who make unexpected decisions that lead to humorous and chaotic events. The plot takes off when one sister impulsively agrees to a wedding that may not be what she truly wants, reflecting on the themes of love, commitment, and self-discovery. The light-hearted tone of the book serves to highlight the ridiculous situations that arise from their choices, making readers laugh as they navigate the complexities of relationships.

The phrase "Underpants! Underpants!" emphasizes the humorous and absurd moments that pop up during the characters’ adventures. This playful element captures the spirit of the book, where misunderstandings and awkward scenarios unfold, leading to both personal growth and comedic relief. Kinsella's writing style is engaging, ensuring that readers stay invested in the characters' journeys while enjoying the fun and chaotic ride of love and life.