Charlotte Mielswetzski's reward for saving the world is getting ultra-mega-grounded. Turns out, it's hard to explain adventures in the Underworld to parents. Zee's managed to evade punishment, instead being treated like he's about to break, but he's been distant since the cousins returned.

When Charlotte's parents announce they're going on a cruise, she thinks things might be looking up. But the cruise is strictly educational...and was orchestrated by Poseidon. Philonecron is Poseidon's grandson and isn't pleased about his delicious plans being thwarted by mortal children, and the god of the sea is only too happy to help him get revenge.

Stranded on the high seas, Charlotte and Zee are squarely in the domain of the second-most powerful god in the universe--who's out for their blood.