The Red Herring Mystery

As an artist, my eyes are trained to observe the minutest detail, enabling me to spot the most obscure clues. Can you do the same? By looking carefully at the notes and pictures in my casebook, can you find all the clues and deduce how the crime has been committed? The reader, as assistant detective, must gather clues from the text and illustrations to help solve a mystery involving a stately home full of possible suspects and a disappearing fish-shaped ruby.