Murder of a lady

Duchlan Castle is a gloomy, forbidding place in the Scottish Highlands. Late one night, the body of the laird's sister is found in the castle. She has been stabbed to death in her bedroom - but the room is locked from within and the windows are barred. Inspector Dundas is dispatched to the case, but he soon has more than one apparently impossible murder to solve. Luckily, help is on hand in the form of gifted amateur sleuth Eustace Hailey. Can he unravel a logical solution to this most fiendish of plots?