The Siren Song picks up a few months after The Shadow Thieves left off. Life has returned to normal for Charlotte and Zee--as normal as they can be when you're the only two people around who know that Greek myths are real. But all is not as quiet as it might seem. For Philonecron is in the Upperworld now, and he's none too happy about the cousins foiling his plot. He has plans for Charlotte and Zee, plans that will lead them to the treacherous realm of his grandfather--the vengeful Poseidon, Lord of the Seas. This time, it's not the world the cousins must save, but themselves.