Not to be confused with GURPS Atomic Horror
In this world, humanity is threatened by the Dragon Emperor who is waging a secret war against mankind from his undersea kingdom. The Dragon Emperor is furious over human incursions into the ocean, culminating in the Bikini atoll nuclear tests. He wishes to destroy man's cities and technology, so humans can't threaten his realm and he can walk among in human guise as he did centuries ago.
The Dragon Emperor has been sending various monsters against human cities, sometime huge, others small. Humans having been killing the monsters, but they don't understand the cause.