A tabletop role-playing book that adapts the Cthulhu Mythos to an investigative game system, emphasizing finding and interpreting clues over repeated skill checks; it provides rules for Purist and Pulp styles of play, mechanics for Stability and long-term Sanity, occult magic and Mythos powers, plus detailed enemies, artifacts, scenarios and Keeper guidance—often grounded in 1930s-period research—to enable tense, detective-style campaigns of cosmic horror.