Prince Dakkar and his mentor Count Oginski discover a plot by arch-enemy Cryptos to kill Napoleon. Arriving on their revolutionary submersible to intercept Cryptos, they glimpse a terrifying monster that seems to escape back into the bowels of the Earth. It leads them to discover an amazing underground world, and a plan more nefarious than they could ever have believed - even from Cryptos.

The stage is set for an epic showdown complete with a giant reptilian cavalry and the Battle of Waterloo, in another breathlessly paced and endlessly inventive adventure for fans of Percy Jackson.