The teens are played, with varying degrees of success, by three young actors (Andrew Ashby, Shelley Crowhurst & Richard Willis) who seem to have all faded into obscurity immediately following their 6 episodes of fame. Each though makes the most of what they have and attack their roles with gusto with particular kudos going to Shelley Crowhurst who as Lucy has to endure some very dodgy gender politics and comes through it all not only as probably the action hero of the piece but with her strident west country accent and matter of fact manners as the comic relief also.
Simon MacCorkindale - a year away from his tussles with the Planet People in Quatermass - Frederick Jaeger - a Doctor Who regular, most notably as K9's creator, Professor Marius - Peter Vaughan - Porridge's Harry Grout - and Wyrd Britain TV legend, John Woodnutt - who appeared in four Doctor Who serials, The Tomorrow People, The Boy From Space & Children of the Stones.
Don Houghton, who had already written scripts for 'Ace of Wands', Doctor Who ('Inferno' & 'Mind of Evil'), several early 70s Hammer movies and would later go on to co-write Assignment 5 of Sapphire & Steel (the one that looks like a Poirot story). Here he revisits the military themed science fiction of his Third Doctor stories, particularly 'Mind of Evil', in a story that sends our intrepid trio scurrying around the South West of England and the Brecon Beacons searching for a missing weapons system that allows the producers to make use of lots of cheap locations and save their money to invest in an impressive final act.