In The Case of the Ruthless Murderers, Mr. Keen, the renowned investigator, finds himself embroiled in a tense situation when Rod Marble, a man he helped convict five years ago, shows up at his office. Marble, recently released from prison, expresses his desire to know who betrayed him to the police, leading to his arrest. Despite his seemingly calm demeanor, Marble's underlying menace is palpable, especially given his past threats against Keen's life. Keen remains steadfast, refusing to divulge any information, which only intensifies Marble's determination to seek revenge.