On the edge of a nebula, the TARDIS lands on the strange planet of Foss, which is covered in dense and intricate minerals and vegetation. The spindly, insect-like Fossians are suspicious of the Doctor and Clara, believing them to be on the side of the large, spider-like Drak-Arzin. But when the travellers meet the Drak-Arzin they discover that Foss is far more than a planet: it is, in fact, a giant life-form, nearing the end of its life-span. But what secret lies at the the heart of the Fossians’ mine? With the help of a young Fossian named Anura, the Doctor and Clara try to intermediate between Foss and its two warring peoples.