Sapphire & Steel (1979)

Season 3: Episode 6

After Silver's return, the three time agents question the couple from the future about their origins and their knowledge about the machine that transported them through time. The couple reveals some horrifying information about the future that directly relates to the creature inside the wall. Now with an understanding of the creature's motivation, Sapphire and Steel must prevent the creature's escape and manipulate time in order to save all of humanity.