While brothers Lexington and Socrates Smith, authority on fingerprints and blood stains, are guests of Peter Mandle and his stepdaughter, they observe a light flashing from the direction of Mr. Jethroe's house. „Come three oaks,” it spells in Morse code. A ghostly figure is seen hurrying across the moonlit lawn. Early next morning the brothers take a stroll, and there, tied to an oak branch, is a body with a purple mark where the bullet struck.