That terrible old reprobate and transdimensional adventuress Iris Wildthyme has gone and locked herself up in a nunnery on the savage world of Artaris. Here she is discovered by the Doctor and the reasonably brutal warlord Grayvorn. Together — with a peculiar nun they pick up en route — they must travel the forests and swamps of this ghastly world in Iris's double decker bus in order to tussle — to the death — with a horde of flesh-eating zombies for a mysterious and holy relic of unfathomable value and questionable pedigree. But what is the Mother Superior's part in all this?