The diplomat Sir George Ramsden is returning to Britain from France with the mysterious "unicorn" in tow. The legendary thief Flamande has declared that he will be on the same flight as Ramsden, in disguise, and that the unicorn will be his. His arch-rival and head of the Srete Nationale, Gasquet, has assured the public that he too will be on the plane to thwart his nemesis.
Meanwhile, holidaying in Paris, the ex-spy Kenwood Blake runs into Evelyn Cheyne and is swept into a perilous chase ending at the Chateau de l'Ile on a stormy night. Here, Ramsden's plane has made an emergency landing, and Henry Merrivale has joined the party. When the castle is cut off by the flooding river, the stage is set for a battle of wits between two masters of disguise in Flamande and Gasquet, as a bizarre and seemingly impossible murder among the party casts suspicion in every direction and the mystery of the unicorn is revealed.
Carter Dickson's brilliantly intricate mystery was first published in Britain in 1936; it remains a testament to his unique talent for wrangling audacious levels of devilishness into a masterpiece.