Sixteen-year-old Terry Connor is sent, along with a few of his friends, to an Outward Bound centre. On his first day at the centre Terry is taken pot-holing by the senior instructor, Alex. All goes well until, at 100 feet underground, Alex goes to search for the torch that Terry has dropped.

Hours pass, and, to Terry's astonishment, when Alex finally returns he has no recollection whatever of having been absent. Terry suspects something sinister is taking place – it is surely no coincidence that there is a secret Ministry of Defence establishment nearby. But just how deeply his curiosity will involve him in dangerous matters becomes clear when he learns the truth about the ‘Jensen Code'.