Rare books and manuscripts have disappeared from important Swiss and Italian libraries. A vague tip from an arrested fence in Amsterdam seems to implicate someone within the English community at Zurich. Scotland Yard sends Inspector George Mason there, to assist the Swiss police in their investigations. Using the cover of scout for a tour company, he slips easily into the English community and moves around Switzerland visiting antiquarian book fairs and bookstores, in the expectation that the missing titles may turn up for sale. Count Flavio, an Italian vintner, is now introduced, traveling to Bellinzona, in the Italian-speaking canton of Ticino, to arrange the program for an important meeting of Amadeo, a secret society destined to play a key role in Italian affairs. The Swiss police eventually arrest a college student, Erik Muntener, for the theft of books at a major university library. Muntener has links to the English community, which is centered on St. Wilfrid's Church. George Mason joins the church choir and befriends the leading bass singer, Max Fifield, and his American girlfriend, Jill Crabtree. He gets Scotland Yard to check the backgrounds of leading members of the choir and discovers that Fifield was involved in extreme right-wing politics before leaving England to live in Switzerland. Fifield is professionally involved in the book trade, as a sales representative for an academic book publisher at Bern. Mason discovers a link between Fifield and Anton Ziegler, a Zurich book dealer with fascist sympathies, who is also a leading member of Amadeo, for whom he acts as main fund-raiser. Inspector Mason, working alongside Leutnant Rolf Kubler of the Zurich police, begins to suspect that this case has much wider implications than the theft of rare books, which is one of several implied sources of finance. His investigations range widely across Switzerland and Northern Italy, introducing the reader to interesting aspects of Swiss life and customs, in his attempt to penetrate Amadeo and expose its sinister objectives.