Biggles goes to the frontier of Tibet to help his professor uncle Dickpa discover the secret of the Mountain of Light. They find themselves in deadly peril at the hands of the Chungs, a strange race who guard the mountain.

goodreads summary

Whilst celebrating Ginger's award of his pilot's licence, an SOS on B.B.C. radio asks for Major James Bigglesworth to contact his Uncle, Professor Richard Bigglesworth. This is of course, Dickpa, previously seen in the story 'The Cruise of the Condor'. Visiting Dickpa at his house, they find him and his friend Lord Roger Maltenham, under attack from mysterious enemies.

This story takes Biggles to Tibet before the Chinese occupation.