Through a Brief Darkness — Richard Peck

What started as a dream come true has turned into a horrible nightmare.

At first it seems like an adventure in a movie. Glamorous relatives pull Karen out of boarding school and whisk her away to their mansion in England. But soon she realizes there's something sinister about her "long-lost cousins." And why is her father suddenly impossible to contact? By the time Karen figures out what's going on, it's too late to turn back -- and she is fighting for her life.