 PLOT: The Cybermen archive contains information about Cybermens  encounter with the Fourth Doctor. The episode “Revenge Of The  Cybermen” takes place in their future. Its placement suggests that  “Revenge Of The Cybermen” took place before “Earthshock”, which is  the year 2526. According to the Doctor in “The Ark In Space”, when  he first steps out onto the station, the Nerva Beacon is 29th or  30th century technology. Viewers have speculated that the “Revenge  Of The Cybermen” clip is possible because the Cybermen had travelled  back in time to prevent the Cyber Wars 