This novel -- and it's really a novel chopped in three, not three novels in a sequence, ending with cliffhangers and all -- opens in a boarding school. Five children kept prisoner and raised there. They know there is something odd about themselves. And not just because when they manage to talk to children outside the school, they learn that they are being given a very unusual education, and not just because they do not know and can not figure out anything about their families and where they came from. For one thing, odd things happen about them, like being able to find a secret passageway some of the time. For another, they do not know how old they are, and the staff of the school are quite careful to keep them from finding out.
For a third, once Amelia found a box of papers telling of stories, but when she looked at them later, she realized they were in her handwriting. Except that she had not been able to write that well when she found them. And they told the story of something the five children did that none of them could remember.
When, however, the Board of Directors meets, and Amelia and Quentin get free to sneak about and spy, they learn quite a bit more about who is keeping them there. And that they are hostages. And from that they work, through many setbacks, toward escape.
An allusive work. I don't think I got all the Greek mythology references, and I know quite a bit of Greek mythology. And Norse mythology, and Beowulf -- and Mother Goose. Among others.