Earth faces imminent destruction. Nemesis, a comet containing a statue composed of validium (living metal) is due to strike, needing only its bow and arrow to achieve critical mass and become active. Four converging parties know of its power: The Doctor, Nazis assault troops from South America, a 17th century noblewoman steeped in the black arts (in whose image the Nemesis statue was fashioned), and discreet but lethal extra-terrestrials.