UFO: The Long Sleep (Episode 26)

Ten years earlier Straker accidentally knocked Catherine Frazer down in his car as she was running away from something which frightened her and she has been unconscious ever since. Now Catherine awakes from her coma and explains that she and a boy named Tim whom she had not long met went to a derelict farm-house to take recreational drugs. There they saw two aliens planting a bomb. The couple ran off - Catherine into the path of Straker's car, Tim, she understood to have fallen off a roof and died. However, he was taken by the aliens who took him over and is now back, looking no older than he did ten years ago and about to set off the bomb.