Excelis Dawns (audio story)

Publisher's Summary
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?