Undermind

Personnel Officer Drew Heriot returns to England after working in Australia to discover his brother 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 the authorities to the danger...