InThe Case of the Phone Box Murder, Hans Muller was a safe-blower who was found murdered in a phone box in London. While at the crime scene with Gaskin, Biggles found two important clues: a piece of paper torn from a Viennese restaurant menu card with the name "Die Blau...." and a square piece of a strange, unfamiliar, metal.
These clues helped Biggles to piece together the circumstances of Muller's death. From the date on the menu card, Biggles surmised that Muller must have flown into Britain by air as there was no other way he could come in so quickly. The piece of metal was thought by Biggles to be a new alloy called crystallium, some of which had just been stolen from a safe at a United Nations research station in Austria.
Following these clues, Biggles and co. proceeded to Vienna where they located an illegal air transportation gang which specialised in moving criminals across borders. The gang operated out of the Blue Goose cafe in Vienna. Customers interested in the air transport service would ask for "ukranean borsch" at the cafe and then present a "ticket" by tearing off a corner of the day's menu card with the word "ukranean borsch" on it. Biggles infiltrated the network undercover using this procedure and met an American who was the leader of the air transportation gang. He told Biggles that it was he who had organised the robbery of crystallium from the research station using Muller. However, Muller had doublecrossed him and had given him a normal piece of nickel, while flying off to Britain with the valuable crystallium himself. Naturally the American had arranged for Muller to meet with an "accident".
Biggles, pretending he was interested in air transportation, paid for and obtained passage to Britain via a De Havilland Puss Moth. On arrival in Britain, he was met by two gang members whom he took to his Mount Street flat to fetch the money to pay for the flight. There, the gang members found Gaskin waiting and were promptly arrested. One of the two gang members had his fingerprints on the knife which had killed Muller and he was subsequently tried and executed for his murder.