The sleepover is intended to cement Megan's new friendship with Rachel, Claire, and Harriet. They would do the usual stuff--talking till the wee hours, painting each other's nails, gossiping. The Parthenon, the huge manor she and her father live in, is even supposed to be haunted so...seance! But Megan stumbles on a loose hearthstone, revealing an old diary and a mysterious key. Before the night is out, the girls are on a dangerous journey into the myths of ancient Greece, and Megan's job is to make sure everyone gets back alive despite gorgons, sea monsters, and sphinxes.