In this two-part episode that opened the 22nd season of the British sci-fi series, the time-traveling doctor (Colin Baker) and his American companion Peri (Nicola Bryant) descend into the stinky depths of London's sewers to find out who (or what) attacked two workers. Not surprisingly, their sleuthing journey takes them across time and space as they pursue jewel thieves and try to abort a plot to change the history of the Earth.