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 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?