The Doctor and Bernice visit Betrushia, a planet famous for its beautiful ring system. They soon discover that the rain-drenched jungles are in turmoil. A vicious, genocidal war is raging between the lizard-like natives. The ground itself is wracked by mysterious earthquakes. And an unknown force is moving inexorably forwards, devastating everything in its path. Ace wanted out; she's resting on a neighbouring world. But from the outer reaches of space, a far greater threat is approaching Betrushia, and even Ace may find it impossible to escape.