The Orpheus' team of scientists are on a mission to find a renewable source of energy in the depths of the Arctic Ocean. The last team who went died in mysterious circumstances, so the Orpheus crew are already feeling nervous when a salvage inspector unexpectedly joins their mission. What does he know about the lost crew, and who is he working for?

The Orpheus crew's quest for renewable energy has led them into danger. They're trapped on Volos, a Russian submarine, and its nuclear reactor is dangerously overheating. Whoever shuts it down will be exposed to lethal radiation. Who will draw the short straw? Meanwhile Clem is determined to find out what part the Volos had in his wife's death.

The Orpheus is mended, and the captain wants the crew to find more of the rare bio-fuel samples before they return to the surface. The Russian captain has other ideas, however, and instructs his men to kill the crew. In their struggle to escape, some are left behind; will they be able to complete their mission?

The Orpheus crew have found rare bio fuels in the ocean depths, but they must find a way of getting it to the surface safely. With the Russian captain of the Volos submarine determined to stop them, will they manage to stabilise the energy source and make it back to the surface?