Jonah and Katherine find themselves flung back into medieval England along with Chip and their new friend Alex.
JB the time traveler sends them back on a mission to fix some loose ends in history. It seems that Chip and Alex were kidnapped out of their own time, and history has to be returned to its proper course by them finishing out the life they should have led.
Chip's original identity was Edward the V of England and Alex was really his younger brother Richard, Duke of York. Technically, at the time the children arrive in England, Chip is the king of the whole country. However, his scheming uncle determines to become king in Edward's place.
If you are a student of history, you will recognize this story as being based on the two princes in the Tower of London who mysteriously disappeared. It is thought that they were murdered by Richard III or that they died of something natural while they were in the Tower. The bones of two children were discovered buried in the Tower of London, but no one can prove whose bones they were. Hence one of the great disappearance mysteries of history.
This is an excellent, page-turning story. I'm on to the next book in the series. My compliments to Margaret Haddix.