In The Rifle Case, Richard Diamond, a private detective, finds himself embroiled in a high-stakes situation when Gustav Heiden, a German gunmaker, seeks his assistance. Heiden, who has recently escaped the horrors of Nazi Germany, has developed a revolutionary rifle that he believes could change the landscape of weaponry. However, his invention has already attracted unwanted attention, leading to an attempted theft. Heiden's request for protection is urgent; he fears for his life and the safety of his family, particularly his sister and her son, who have helped him settle in America.