The Doctor, Jamie and Zoe enter the TARDIS, watched with bemusement by Isobel and UNIT's Captain Turner. Lethbridge-Stewart reappears, now promoted to Brigadier. UNIT was created in Geneva, with the Brigadier in charge of British forces [dialogue in Spearhead from Space would seem to indicate that UNIT was already in existence at this time, Lethbridge-Stewart joining later].The Cyber Director tells Vaughn that the Doctor and Jamie have been recognised 'from Planet 14' [an untelevised adventure, as all of the second Doctor's other meetings with the Cybermen take place, chronologically, after this date, and the Cybermen as yet show no time travel ability] Peter Halliday also provided, uncredited, the voices of the Cybermen and of the Cyber Director hidden in a concealed compartment in Vaughn's office.Watkins is being forced by Vaughn to develop the cerebration mentor, a machine designed to generate emotional impulses as a weapon against the Cybermen.The Cybermen immobilise most of Earth's population - sending a hypnotic signal through special circuits incorporated in all IE equipment - and launch their invasion.To avoid capture by guards, the Doctor and Jamie each hide inside a crate loaded aboard the IE train.Jamie finds himself lying next to some sort of a cocoon.
Stock footage of radar dishes, missiles etc is employed extensively.
On the closing credits of Episode Five Ralph Carrigan's surname was misspelt 'Carrigon'.
The TARDIS becomes invisible for the first time after materialising.
Zoe makes no appearance in Episode Four of the story as Wendy Padbury was on holiday during the week when it was recorded.
Similarly Jamie appears only in a pre-filmed insert in Episode Eight as Frazer Hines was due for a break.