Meet the Blanchard twins:

Rose is practical, sharp, and protective. Specialty: manta rays.

Finn is too sweet and kind for his own good. Specialty: sharks.

Rose and Finn are looking for sharks on the sea bed. When they return to their ship, the Dauphin, they make a terrible discovery: their colleague has been murdered on the ice - with a harpoon that should never have been there. Everyone else on the luxury cruise ship - the wealthy passengers, the researchers hard at work, the tight-knit crew and their strangely calm Captain - were all aboard at the time, so they are all under suspicion. Rose and Finn were the only two at sea, and they're miles from anywhere, so it's up to them to investigate.

On scientific problems, they are a brilliant team - but can the Blanchard twins solve a murder?