The article reveals the names of the prototypes of certain characters in Nagibin’s long story My Golden Mother-in-Law [Moya zolotaya tyoshcha] (the mother-in-law being A. Likhachyova, the wife of the director of the Moscow Car Manufacturing Plant ZIL). For the first time we read the names and learn about the destiny of M. and L. Kostromin, the real people behind the characters of Matvey Matveevich, the neighbour, and Nina Petrovna, the female protagonist’s best friend. The life story of these personalities, residents of the legendary Niernsee House in Bolshoy Gnezdnikovsky Lane, enables the authors, who are related to L. Kostromina, to explain the underpinnings of the relationships between the prototypes of Nagibin’s characters, often a mystery for the writer himself, and share first-hand accounts that confirm his amazing flair for imagination. The article uses materials of family lore, the authors’ private archive (letters and photographs), as well as hitherto unsearched materials from state archives.