A murder has taken place on stage and it seems that one of three people must be guilty. The crime was committed in full view of the audience and players, but no one can say whom the murderer is. There appear to be no clues, the suspects are all well trained in the art of dissimulation, and all three deny any knowledge of the crime.

It looks like the perfect murder, until Dr Basil Willing, psychiatrist-sleuth, begins to investigate the peculiar behaviour of a pet canary and a housefly.