The ghostly visitors

Catherine Blake comes to a huge mansion in the remote English countryside as governess to Miles,10, and his sister Flora, 7. The children seem to be angelic and innocent, but Catherine soon discovers that this is not the case. Then two ghostly visitors, a man and a woman, start appearing. They seem to be watching the children, and the children seem to be aware of their presence. What is going on? Who are the ghostly visitors and what do they want from the children?