The Valley of Adventure by Enid Blyton

When Jack, Philip, Dinah, and Lucy-Ann mistakenly climb aboard a strange plane, they find themselves whisked away to a secluded valley ringed by mountains. After befriending an elderly couple guarding a hidden treasure, the children encounter a band of thieves seeking to plunder the valley of its riches. Dodging peril at every turn, the intrepid foursome embark on thrilling adventures underground and high in the mountains.

With new friends by their side, can they outwit their foes and protect the valley's long-lost treasures before it’s too late?