Serial: 4F  Episode Nos. 402, 403, 404, and 405.  Title: Terror of the Zygons

UNIT is investigating the destruction of oil rigs in the North Sea, and the Brigadier recalls the Doctor to Earth to help.  Their alien enemies soon target him for destruction, sending disguised members of their own race to destroy him, and also a much larger armored cyborg known better as the Loch Ness Monster...

This story sees the departure of Ian Marter as Harry Sullivan (as a regular cast member).

starring Tom Baker as Doctor Who, Elisabeth Sladen as Sarah Jane Smith,  Ian Marter as Harry Sullivan, Nicholas Courtney as Brigadier Lethbridge-Stewart, and John Levene as RSM Benton. Written by Robert Banks Stewart, Produced by Phillip Hinchcliffe, and Directed by Douglas Camfield.

Originally transmitted from 30 August 1975 to 20 September 1975 on BBC1.