One normal night over the Scottish highlands, Flt. Lt. Chris Drake (Vincent Regan) was scrambled and shot down an unidentified flying object little knowing the consequences of this rash act of bravado. At the same time Dr. Amanda Tucker ('Maggie ONeill) and her assistant Nick Shay ('Paul Shay' ) were monitoring the sky for intelligent forms of communication and detected much more than they bargained for. The search for the downed aircraft led to the discovery of Lt. Charles Terrell (Anton Lesser), who subsequently turned out to be an army officer who went missing in 1945. After initial reticence Lt. Terrell started to reveal his story about the peaceable Echo's who had abducted him years ago and an ancient, merciless enemy, the nD's, who sought to enslave the Echo's for their own evil purposes. As his story unfolds and the evidence of Dr. Tucker's observations and the construction of the alien spacecraft lends credence to it, the nightmarish prospect of a full scale invasion of the planet Earth starts to become a terrifying reality. Standing against this formidable enemy, that Lt. Terrell is convinced will prevail, are only Major Gen. David Reece ('Fred Ward' ) and the men of his command. Neither he nor his second in command Squadron Ldr. Helen Knox ('Phyllis Logan' ) are able to convince their superior officers of the dire situation the Earth faces.