While visiting their cousin Sherlock in London, Emily and Alexander get caught in the middle of an exciting criminal case. The famous writer Arthur Conan Doyle has had his manuscript "The Hound of the Baskervilles" stolen. The ashes in the fireplace and scraps of paper indicate that it was burned. Or did someone want to lay a false trail here? The three clever mice start their own investigation and soon get on the trail of the real culprit.