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... (thetvdb.com)