Simon and Mary fake a crash landing in the Antarctic to get them into a research base there. A message from a Centauran was sent from the base. An observer sent in to find the source of the signal was killed in a sledging ""accident"". Simon and Mary arrive to find that the radio has been destroyed in an accident and the only contact with the outside world is a plane that will be arriving in a few days. This confirms to Simon that one of the men on the base is a Centauran. The question is, which on is it...? (thetvdb.com)