Sherlock Holmes. The Speckled Band: Dominoes Starter

'Help me,' says Helen Stoner. 'My sister died, and now I am going to die too.' Sherlock Holmes and Or Watson must answer the question of how Julia Stoner died, two years ago. If they cannot, then Helen Stoner will die too. But her house is very strange -and her stepfather, Or Grimesby Roylott, is very angry and does not want Holmes there.
Who killed Julia, and how? And why did she say 'the speckled band!' before she died?