Counterstrike (UK)

Sent to Earth to protect it from invasion, Simon King comes across news stories about unidentified objects seen over Penfield, Sussex which he believes could be the start of some new activity. Simon's investigations take him to a factory who are producing a revolutionary type of transistor radio. Simon visits the factory posing as a journalist. His visit worries the owners of the factory who look into his background. Inconsistencies there make them nervous. Later that evening Simon is shot and rushed to hospital. Mary, the Doctor in casualty takes a blood sample but the results come back that the blood isn't from a human. Then King sneaks out of the hospital. Mary follows him to his home and what she finds there turns her world upside down...

Peter, one of the North American agents is killed as he sends a desperate message to Simon. Simon and Mary go to New York to investigate. On arrival at the apartment Mary finds what she thinks is Simon's dead body. However, the body is Peter, as most of the agents on Earth look like Simon. Simon sees this as confirmation that whoever they are up against knows they are there and are laying a trap. Peter was posing as Doctor Westhoff, a member of a Think Tank set up to discuss ways of reducing the worlds population. Simon decides to spring the trap and takes Peter's place in the Think Tank...

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...?