Twelve-year-old February Callendar and her older brother Friday are home from boarding school for the summer. For fun, Friday has been digging a tunnel through the backyard. When their father, a war correspondent, leaves to cover an international crisis, the siblings escape their mother and their tutor and get wrapped up in a crisis of their own. They can hardly believe it when their search for clues -- including a suspicious plane crash, a mysterious mineral, and a comic strip with secret messages -- leads right back to Fridays tunnel and a chance to save the world during their summer holiday.