Sergeant Nix Marr left her dark past buried beneath the sea. Then Kessandra, the people's favourite princess (and Nix's much-loathed ex), recruits Nix for a covert mission. Their goal - to investigate a massacre in the under-water city of Fall. Nix tries to refuse, but finds herself boarding a luxurious submersible as Kessandra's bodyguard. After all, Kess always gets what she wants.

As they descend to the city, Kessandra reveals the chilling truth. They aren't investigating a massacre, but the trigger that caused it - a sickness that turns victims into mindless killers. And when a royal passenger is murdered, it's clear the infection is on board . . . and no one on the submersible is safe.