The Beast Must Die is a powerful story of vengeance and retribution. Herbert Marshall plays the part of Felix Laine, a writer by profession of detective novels, who was planning to kill a man. He didn't know his name, where he lived or what he looked like but he knew that he was going to find him and kill him. He recorded all of the details in a diary, which was either the best murder novel he had ever written, or an incriminating document.