With the first segment successfully retrieved, the Doctor, Romana and K-9 trace the second segment of the Key to the planet Calufrax. But when they arrive at exactly the right point in space, they find themselves on exactly the wrong planet - Zanak.

Ruled by the mysterious 'Captain', Zanak is a happy and prosperous planet. Mostly. If the mines run out of valuable minerals and gems then the Captain merely announces a New Golden Age and they fill up again. It's an economic miracle - so obviously something's very wrong...