"When We Believed in Mermaids" tells the story of Kit, a woman who believes her sister Josie died years ago in a terrorist attack, but then sees a woman resembling her on the news, leading her to travel to New Zealand to uncover the truth about her sister's life, confronting long-buried secrets and a tumultuous past as she tries to reconnect with Josie, who has been living a new life under a different identity.