There was a prince who was about 10 years old. To prove him a worthy heir to the throne, the king would erase part of his memory once a week, and he would have to use hints provided by his mother and sister to figure out what he can't remember. If he didn't remember, he would be put to death. His sister would always mislead him, but his mother always helped him. His mother had magic powers and a spellbook.
That is all I remember.