S1·E31 The Case of the Exhumed Client

Sir Charles Farnsworth is found dead in his mysterious Farnsworth Castle. It turns out that Farnsworth had a clause inserted in this will that his death, no matter what the apparent cause, would be investigated by Sherlock Holmes. Holmes' investigation reveals traces of arsenic in the man's body, and there seems to be no shortage of people who knew Sir Charles who wanted him dead.