Serial: WWW  Episode Nos. 360, 361, 362, 363, 364, and 365.  Title: Invasion of the Dinosaurs

Upon returning to London in the present-day, the Doctor and Sarah find it totally deserted apart from small gangs of looters, military police patrols, and dinosaurs!  At least the dinosaurs seem to vanish before they do too much damage, but then others turn up, and no one has a clue as to where they come from or go to.  The Doctor soon has some theories, of course, and rapidly finds he’s getting too close to the truth for those involved in a conspiracy that has infiltrated UNIT itself...

starring Jon Pertwee as Doctor Who, Elisabeth Sladen as Sarah Jane Smith, Nicholas Courtney as Brigadier Lethbridge-Stewart, Richard Franklin as Captain Yates, and John Levene as Sergeant Benton. Written by Malcolm Hulke, Produced by Barry Letts, and Directed by Paddy Russell.

Originally transmitted from 12 January 1974 to 16 February 1974 on BBC1.
 
The first episode of this story is titled simply Invasion and is presented on this videotape in black and white as that is the only form in which the BBC Archives hold a copy.  This first episode was only rarely broadcast on North American television.