The Improbable Prisoner (The Further Adventures of Sherlock Holmes)

Following a summons to a fictional patient, Dr. Watson finds himself arrested for the horrific murder of an elderly woman. Imprisoned at Holloway, and with the evidence stacked against him, Watson's only hope is that Sherlock Holmes can discover the identity of the real killer. But when a mysterious letter appears to link Watson to blackmail and a notorious street gang, Holmes must use all his powers of reasoning to save his friend from the hangman's noose.