The famous writer Philip Boyce died of arsenic poisoning. The police have no doubt that the murder was committed by the man's ex-girlfriend Harriet Vane, also a writer. Everything points to her guilt, even the fact that at that moment she was writing a book about arsenic poisoning. No one doubts that the court will find Harriet guilty, not even herself. Despite the fact that everything looks very convincing and irrefutable, Peter Wimsey knows that the girl is innocent. But who, if not her?