Inspector and Fish

Inspector Robert Cassidy, one of the greatest detectives of a generation, only coming second to a certain Sherlock. Unlucky, not popular or successful, after the humiliation of his unspoken last case, The inspector is called to an unsolvable mystery in a house full of strange shadows, where only the walls seem to know the truth. Can the not-so-great detective solve the case with only an unresponsive fish for help?