Dr. Arnold, a famous criminologist, disagrees with his associates concerning his pet subject. To prove his point, he disguises himself as a tough and goes into the slums to gather the necessary data. While in a low dive he makes the acquaintance of Luigi, an Italian criminal. Luigi is the cause of a brawl, ending in an affray in which Dr. Arnold and the Italian receive deep knife wounds. The injured men are removed to a hospital. The physician has lost so much blood that his life is despaired of. Luigi agrees to allow his blood to be transferred into Dr. Arnold with the stipulation that he be spared prosecution. The operation is performed and the doctor recovers. He becomes conscious that, with the Italian’s blood, the criminal tendencies were transferred to him. He fights against the thought, but it masters him. To his horror, he finds on returning home one night, that he has stolen a costly necklace from a woman acquaintance. He consults a professional associate and they agree that the only method of saving him will be to perform another operation and inject into his blood a serum which will neutralize the effects of the Italian’s blood. The operation is performed. The taint of the alien blood is found to be removed and he has the right of a good man to ask a good woman to marry him.