The series was devised by Shoestring and Bergerac creator Robert Banks Stewart, who also went on to write for Doctor Who.

Several other writers known for their work on Who also contributed scripts: David Whitaker, Bill Strutton and Robert Holmes.

Personnel Officer Drew Heriot returns to the United Kingdom after working in Australia to discover his brother, Police Detective Frank Heriot, behaving oddly, to the distress of his sister-in-law Anne.

Investigating together, they discover that a disembodied alien force is using high frequency signals to brainwash people – including Drew's brother – into committing subversive acts as a prelude to a full-scale invasion.

Together, Drew and Anne battle to stop the acts of sabotage while trying to alert Britain to the danger.