Doctor Who: The Fate of Krelos (2015)

Something gigantic is descending upon Krelos. While The Doctor and Leela are away fishing in the mountain pools of Krelos, K9 reactivates the Tardis' architectural configuration and there's a familiar piece of material snagged on the console. Far up the mountain, an aged explorer is in trouble. Will The Doctor and Leela be able to save him and this planet? And what is it that K9 has discovered in the Tardis?