St. Anthony's Fire — Mark Gatiss

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 neighboring world. But from the outer reaches of space, a far greater threat is approaching Betrushia, and even Ace may find it impossible to escape.
With time running out, the Doctor must save the people of Betrushia from their own terrible legacy before the wrath of St Anthony's Fire is visited upon them all.