Friday's Tunnel

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 Friday's tunnel
and a chance to save the world during their summer vacation.