The story:
A U.N. archaeological expedition is about to enter the Misty Mountain Cavelands despite earnest warnings from their Tirangi guides. Death and evil is said to reside in the caves, but the scientists penetrate the darkness, only to disappear without a trace.

Receiving a request from the U.N. for help (delivered by Diana in person), the Phantom and Diana also ignore the Tirangi and start exploring the caves for any signs of the lost expedition, unaware that they soon will be disappearing too...