In Secret

Rachel and Calliope have two separate lives, one as Muggles, one as witches. They use a Time Turner to go to both Muggle school and Hogwarts. The two identities have nothing to do with each other, until one day, a strange envelope is delivered to them, bearing, on the inside, one letter, "N". This leads to a strange trail of letters, which make a sentence that the girls must decode; before it's too late....