The Wheel. A ring of ice and metal turning around a moon of Saturn, home to a mining colony supplying a resource-hungry Earth--a colony plagued by problems. Equipment failures and thefts are on the rise. Children tell stories of mysterious creatures glimpsed aboard the Wheel. And some of the younger workers refuse to go down into the warren-like mines.

And then one of them, surfing Saturn's rings, saves an enigmatic blue box from destruction.

Once on the Wheel, the Doctor and his companions, Jamie and Zoe, face a critical situation when they become suspected by some as the source of the ongoing sabotage. They soon find themselves caught in a mystery that reaches back to the creation of the solar system. A mystery that could destroy the Wheel--and kill them all...