Quentin Lawrence would later work on the 1958 film version (which was scripted by Jimmy Sangster) of this forgotten ATV serial. In the Swiss mountain village of Trollenberg climbers are mysteriously disappearing on the mountainside. Meanwhile an observatory detects a cloud on the mountainside which gives off radioactive readings and never moves.

One of two English sisters who form a psychic stage act receives a premonition of great danger associated with the mountain. As the cloud starts to move down the mountain, killing people along the way, the astronomers realise they are dealing with an invasion of aliens who have come from a dying world and have selected the mountainside’s cold as the environment which most resembles their home planet.

The film version made in 1958 by Eros Productions increased the gore quota considerably and is also known as The Crawling Eye in America.

Action TV received an email from Ric Cooper in January 2006 which shed some intreesting details on the writing of the series: "Peter Key, credited as the writer, was in fact a team of three: George Kerr, Jack Cross, and my father Giles Cooper. They set up Peter Key as a limited
company, a daring thing to do in the '50s, but it ended up as our family company when my father started achieving serious success on TV under his own name, with Maigret, and then various classic series and adaptations such as Sword of Honour and Madame Bovary, as well as his own original plays including Unman Wittering & Zigo".