For over fifteen years the crew of Kollo-Zarnista Mining Facility 27 has been extracting diamonds from deep within the atmosphere of Saturn, diamonds that help to fund the ever-expanding Human Empire. But when a mining operation goes wrong, a rescue mission must be launched to save a worker lost overboard, a worker who claims that he has seen something amongst the swirling clouds. Something that can't possibly exist.

When the Doctor and Bill arrive, they immediately find themselves caught between hostile miners, suspicious security guards and corrupt company officials as they face accusations of sabotage and diamond theft.

And below them, in the crushing atmosphere of the gas giant, something is starting to rise.