Holmes discovered where the letter came from, it was from a place called Dartmoor,In order to continue investigating the case, he decided to head towards there, and make clearer all the rumors.
in the town there was a laboratory which was known to carry out experiments on animals that were also very risky, so Holmes wanted to get in there.
when Holmes got into Baskerville's lab, he was looking for information that was related with a Hound, but he couldn't find many things
like Holmes didn't get information he decided to go to the moors, and he and Sir.Henry had a Dark meeting with the Hound, Holmes was doubting because watson didn't saw anything and he was confused
Holmes Knew that all of this was related the baskerville's lab and he went undercover to get information about a proyect related with the hound, because he thought that the creature wasn't real but something bad was happening, so then...he found the H.O.U.N.D proyect
it was a proyect that was based in a kind of drug that made that the people saw things that wasn't there, and cause fear to control the people, finally Holmes discovered who did it and the case was closed.