This episode is great. Very original story to bring into the series. The Doctor and Sarah arrive back in England but discover city abandoned under marshall law.
They begin to encounter dinosaur creatures like a Pterosaur (redirect from Teradactyl then a Tyrannosaurus Rex appearing out of nowhere.
This is an interesting and original story that wouldn't worked if it wasn't for the writers, actors, and director of the episode.
One of those Doctor Who episodes that was televised in color, but later only black and white because of the Bbc erasure of the master. Gladly it still exist.
Story-wise, it's an interesting story with Dinosaurs appearing out of nowhere. There source isn't really introduced until the next episode. Seeing how Unit and The Doctor responds to the situation is interesting.
The Dinosaurs were done using primitive effects like miniature models but given the limited technology at the time, they did what they could with what they had.
The story works well given it presents a lot of interesting possibilities with different dinosaurs on display, the unpredictable nature of them appearing in all places, and the fact that The Doctor and Unit are battling them.
It's a great start with them not really revealing much but this is what gets you interested to watch the whole story.