Stealing the sword

"At the flea market nine-and-a-half-year-old Chase and his younger sister Ava acquire an old suitcase filled with rows of carefully packed strange objects, and when they handle one of the objects (a dragon-headed doorknob) they find themselves in King Arthur's castle, on a mission to save the King--and pursued by a strange man whom they first saw at the flea market, who is after their suitcase"--Provided by publisher.