Philip, Dinah, Jack and Lucy-Ann are thrilled when their detective friend Bill invites them on an overnight trip in his new aeroplane. But things go unexpectedly and horribly wrong — and the four children find themselves stranded in a wild, lonely valley, along with a group of men who are clearly up to no good. What secret does the beautiful but apparently inescapable valley hide, and can the children find it before their enemies do?