A new client suffering from amnesia after being attacked arrives in Baker Street and has an intriguing tale to tell. His bandaged fingers are a result of his somnambulism which causes him to scrabble madly on the floorboards at one particular place in his house. Holmes is intrigued and it requires both Holmes and Watson to disguise themselves and assume new and very different personas to ascertain the reason for his clients actions. The discovery of a rare, gold coin from a Spanish shipwreck may be the key to the affair and Holmes devises a trap to solve the case.