When Robert Louis Stevenson wrote Strange Case of Dr Jekyll and Mr Hyde, he did not dedicate it to his wife Fanny, despite her crucial role in persuading him to burn the first draft and write a new version that became an instant best-seller. Instead he wrote a dedication to his cousin, Mrs Katharine de Mattos. Why did Stevenson link Katharine with this dark tale of duality, and what role did she play in its creation? In Mrs Jekyll and Cousin Hyde, Jeremy Hodges tells the story of the cousins' close relationship, from childhood romance to bitter estrangement in later life.