A dinosaur strolls out of the sea wearing red contacts and the British will have none of it. They march to meet it, torches in hand, and treat it like it were a Google van coming into town.
The aquatic monster retreats immediately back out to sea.
After coming across this giant beast, both scientists and circus-operators are very interested. In the end, the guys who want to catch it, exhibit it, and sell tickets end up netting the monster and doing just that. Their fun is short-lived however, since the scientists eventually show up to ruin their fun by explaining that what they've caught is just an infant who's parent is likely to be nearby.
They turn out to be exactly right. The 200 foot mother, Ogra shows up soon, heads straight for London, and trashes all of its iconic buildings. The military is sent to annoy it but doesn't really accomplish anything. Pretty soon Gorgo is rescued. The film ends happily with he and Ogra walking off into the sunset together, going back below the sea.
Out of all the "giant monster attacking the city" movies, this one has a few things that make it unique. For one thing, it actually has a happy ending for all parties involved, including the monsters. Also, it has one of the earliest instances of "random kid popping up to inexplicably give insight into the monster's behavior" that becomes a staple in so many other giant monster films.
And the effects aren't really that bad considering what else is out there.
Available on Amazon: Gorgo (Widescreen Destruction Edition)