The Wheel. A ring of ice and steel around a moon of Saturn, and home to a mining colony supplying Earth. It's a bad place to grow up. The colony has been plagued by problems and there are stories of mysterious creatures glimpsed aboard the Wheel. Many of the younger workers refuse to go down the warren-like mines anymore. And then young Phee Laws, surfing Saturn's rings, saves an enigmatic blue box from destruction. Aboard the Wheel, the Doctor, Jamie and Zoe find themselves caught in a mystery that goes right back to the creation of the solar system. A mystery that could kill them all.