Can someone please tell me the plot of the Baskerville Hound episode?

Wikipedia has a pretty good synopsis link. Basically, the hound they were seeing was the result of a chemical weapon that the people were trying to develop, which made آپ feel terrified and hallucinate things. Despite the project being scrapped, the guy from the government that they were talking to secretly continued working on the project, testing it out in the countryside. Which is why people kept hallucinating the hound.