Ronald Hudson, travel writer and adventurer, visits Philip Tolefree with an anonymous letter written in cypher, which he hopes the sleuth can decode. But Tolefree is uneasy. What are Hudson's motives? Is his writer-persona a facade for espionage - or something even more nefarious? When a chance telephone call links Hudson's visit with the murder of a scientist during a country house party Tolefree's worst suspicions seem confirmed. But what is the secret of Old Hallerden?