Dinosaurs in London, another literal sub-plot and the debut of the Whomobile!
The Third Doctor and not-officially-the-new-companion-yet, Sarah Jane Smith, return to London after their medieval jaunt only to find it overrun with dinosaurs. Big scary dinosaurs. From the past!
Our heroes then spend a chunk of this serial under arrest, escaping, getting caught again and hanging out with UNIT, while mad scientists and a government and military cabal conspire to reset the entire human race. No biggie.