This short study explores detective stories by Crown Prince Vajiravudh, later King Rama VI, in early 20th-century Siam. Compiled in Nithan Thong-in, these tales provide insights into Siamese society, culture, and history. Despite multiple reprints, an English translation is currently unavailable. This notice introduces the cultural richness of Mr Thong-in’s adventures, along with brief summaries of each story.