This might be one of my favourite Enid Blyton books. It's sheer magic!
Jack, Philip, Dinah and Lucy-Ann (not to mention Kiki) are all set to go on a plane trip to their friend, secret agent Bill Cunningham's house for a few days. But a mix-up sees them boarding the wrong plane and falling headlong into another adventure - this time in a beautiful, abandoned, war-torn valley - involving a couple of escaped prisoners, a secret cave, a hostage, a daring rescue mission, a map and lost treasure!
Seriously, who can come up with something like that AND make it believable? They don't make authors like Enid Blyton anymore.