Episodes 1 and 4 of this adventure are missing from the BBC archives, and so Nicholas Courtney (the Brigadier) fills in the narrative. The Doctor (Patrick Troughton), Jamie and Zoe arrive in late 20th century London to discover that something fishy is going on at Tobias Vaughan's electronics factory. A scientist has gone missing, and Brigadier Lethbridge-Stewart and the newly formed UNIT are on the case. What does Vaughan produce in his factory, and who are the creatures he speaks to through that revolving whirring device in his office cupboard? It can only be the Cybermen, out to invade the Earth and convert humankind. Resistance is useless, but the Doctor and UNIT will have a go anyway.