All is not well in the convent atop Mount Excelis, highest mountain in all of Artaris. The new arrival is terribly disruptive, and Sister Jolene eventually complains directly to the Mother Superior. The newcomer cackles inappropriately, listens to loud music, drinks, tells ludicrous tales of her past adventures, and invites the other nuns out on day trips in her ridiculous vehicle. Sister Jolene demands that the newcomer be banished, but the Mother Superior decides instead to tell her that she’s been assigned to an important task. Perhaps then, Sister Iris won’t realise that she’s actually being sent away in disgrace.

Artaris is a primitive, superstitious world, where only the harshest and most ruthless survive. But rumours spread through the land of a mysterious, holy Relic which will lift them all to a brighter destiny... however, nobody is quite sure what the Relic is exactly, and few are worthy of it. One who considers himself worthy is the warlord Grayvorn, who is on his way to the convent of Excelis when he meets a stranger called the Doctor. The Doctor claims to have been transporting a monstrous slug-like creature through time and space; he stopped off here to give his weary time engine a rest. Intrigued, Grayvorn decides to allow the Doctor to accompany him to the convent, although at times it seems as though Grayvorn is the one who is accompanying the Doctor. Together, they approach the nunnery, which was built inside an ancient fortress, the purpose of which has been lost to history. Many civilisations have risen and fallen on Artaris, leaving no trace behind them.

Jolene finds Iris singing to herself as she tends the cabbages in the convent’s garden. Iris is bored out of her mind, but becomes concerned when told that she’s been summoned to see the Mother Superior. What really concerns her, however, is the fact that she doesn’t understand why she this should concern her so much. She’s always been quite a hellraiser; what on earth possessed her to join a convent of all things? Does she really want to become a better person? That doesn’t sound like her at all. Disturbed, Iris goes to speak to the Mother Superior, who eventually manages to get a word in edgeways and explain that she’s also concluded that the contemplative life doesn’t suit Iris at all. What brought Iris to abandon her double-decker bus outside the convent and make a plea for sanctuary? Perhaps she was possessed, perhaps she was enchanted; more likely, she was drunk. The Mother Superior therefore informs Iris that she’s selected her for an important mission. Lord Grayvorn has put the nuns to work sorting through their library for information about the Relic, and at last, they have found a map which reveals its location. Grayvorn cannot be trusted with this information alone, and Iris must therefore accompany him on his quest, to act as the Mother Superior’s informant.

Grayvorn and the Doctor arrive at the convent, where Grayvorn learns to his disgust that two nuns will be accompanying him on his quest whether he likes it or not. The Doctor is put off by Grayvorn’s harsh, hectoring attitude... but he’s even more disturbed when Iris Wildthyme calls out a cheery greeting to him. To Grayvorn’s irritation and the Doctor’s despair, the quest will be undertaken in the number 22 double-decker bus to Putney Common, by Grayvorn, the Doctor, Iris, and Sister Jolene. Grayvorn has no choice, as Iris has the only map.

The road down the mountain is steep, and the travellers are unhappy; the Doctor is sure that Iris will make a bad situation worse, while Iris insists that the Doctor’s just hiding his love for her. She stops off in the town of Excelis to do some shopping, even though the natives of the town are preparing for a festival at which they will sacrifice outsiders to appease their goddess. Grayvorn is humiliated to find himself helping an annoying old harridan to carry her shopping, and when she continues to talk of her old adventures with the Doctor, Grayvorn snaps and drags her into a back alley, where he tries to force her to hand over the map. However, she’s slipped the map to the Doctor. The Doctor and Jolene hear the shouting and intervene; whatever the Doctor really thinks of Iris, he won’t stand for Grayvorn harming her. Jolene, meanwhile, has told the Doctor more about the Relic; while the Doctor suspects that it will turn out to be nothing more than an old statue or vase, Jolene believes that the Relic in fact possesses ancient powers lost to history and that the violent Grayvorn is the only one who can find it and survive. But he must not be allowed to tap its powers for himself...

The travellers leave Excelis before the townspeople start looking for sacrifices. On their way down the slope, the Doctor questions Iris about her presence, and she admits that she doesn’t know why she entered the convent -- but suspects that it’s something to do with the Relic. She starts to reminisce about her old adventures with the Doctor once more, but he has no idea what she’s talking about. Iris, by his side when the Sontarans invaded Gallifrey and when the Time Lords sent him to prevent the creation of the Daleks? Omega, sulking in a B&B in Bournemouth? Iris can be a very dangerous and deluded person to be around at the best of times...

Iris crashes the bus out onto the flat valley floor, and decides to stay where it’s stalled for the night. They’ve got a long and dangerous journey ahead, through perilous forests to the horde of flesh-eating zombies who guard the Relic. According to legend, or to Iris, the Relic permits the zombies to live beyond death; this business is literally all about life and death. Grayvorn is still angered by Iris’ overly chummy and patronising attitude, but the Doctor calms him down; like it or not, they have to work together, and it’s best if they all get along. As yet, none of them know that the Zombie King already knows that they’re on their way; it was foretold long ago, and the Zombie King is in a great tizzy about it, as he fears that these primitives will wrest the beloved Relic away from him...

Over the next few days, the odd fellowship continues through the forest, travelling by day and camping out by night. At one point, Grayvorn is attacked by a sabretooth while collecting firewood, and to Iris’ annoyance, the Doctor commandeers the bus to drive them out of danger. One night, Grayvorn and Jolene fall into a conversation by the campfire. As a child, Grayvorn was lured away from his native village by a charismatic stranger who promised him adventure but instead sold him into slavery. Grayvorn has grown to see life as a great cycle of enslavement and retribution, but he sometimes dreams of more than that; he has allowed the Doctor and Iris to live because he senses that they will play important roles in the destiny of Artaris. When Grayvorn first saw the Doctor arguing with a strange, shrill woman whom he left behind in his “TARDIS”, he sensed that the Doctor had a power which Grayvorn could use to change the future. Grayvorn has dreamt of this future, and believes that he, a simple savage soldier from the middle of nowhere, will be there to see it. But what will Jolene’s destiny be?

The Doctor suggests a shortcut which results in the bus getting stuck in the swamp, and Iris must admit that her dematerialisation circuit is damaged, which is why she didn’t transport them all straight to the heart of the forest in the first place. She reluctantly allows the Doctor to take a look at the damage, but it’s going to take some time to fix. As night falls, the Doctor confesses to Iris that he doesn’t quite trust Jolene. Iris, who considers her a friend, is upset and questions the Doctor’s need to live in a universe of moral absolutes. He used to be much more fun in his last incarnation. The Doctor admits that he blames himself for Adric’s death, but while Iris sympathises, she believes that such a loss would cause her to become even more carefree, determined to have fun out of life while she still can. The Doctor returns to work, leaving Iris to sorrow over what he’s become.

The next morning, however, the Doctor is proven right when he, Iris and Grayvorn awake to find that Sister Jolene has absconded with the map. In the heart of the forest, the zombies chant and mutter incantations while the Zombie King frets fearfully over the approach of those who would defy him and take the Relic. Jolene arrives and introduces herself, claiming that the Mother Superior has sent her to learn the secrets of the Relic; however, the Zombie King will not listen to her. The zombies have used the Relic’s power to hold back death and now live only to protect it, and Sister Jolene and her “friends” will die a thousand times for defying the Zombie King. He may mean this literally.

The Doctor has finished his work, and thus dematerialises the bus. Grayvorn is bewildered and horrified by the interstitial madness of the Time Vortex, but there is worse to come as the bus careers out of control. The Doctor only intended to move a few yards to the left, but instead, the bus hurtles through the centuries and materialises in a blasted wasteland, scorched by some unimaginable fire. They are forced to admit to Grayvorn that this is Artaris, a thousand years in the future. It’s only one possible future of many, but all that Grayvorn understands is that everything he knows and values will be destroyed. As Iris and the Doctor set off back to the swamp, Grayvorn vows to do whatever it takes to save his world. He believes that only the Relic has the power to do that, and Iris also admits that she’s read up on the Relic in the old texts in the nuns’ library. She doesn’t believe it to be a weapon, however; if the old stories can be believed, then it’s something very holy indeed.

The bus materialises in the zombie village, sending the zombies into a panic as the Zombie King gibbers tiresome threats at Jolene. The Doctor stops Grayvorn from attacking, hoping that he can still negotiate peacefully for Jolene’s release, but the Zombie King refuses to listen and takes them all prisoner, vowing to sacrifice them at sundown. Grayvorn is enraged, but the Doctor points out that he couldn’t have killed the zombies with his sword; they would just have risen from the dead once again. Iris is upset with Jolene for betraying them, but nevertheless expands on what she was saying earlier. It’s said that the Relic is the key to the afterlife on Artaris; whoever controls the Relic controls the gates to Heaven and Hell. The Doctor isn’t as sceptical as Iris expected; the last time he regenerated, he met his future self before the event, and recently, when he met his past selves in the Death Zone on Gallifrey, they all seemed older than they should have been, as if they’d somehow gone on living after they “died”. Perhaps there’s more to Death than the Doctor has ever believed.

As the sun sets, the zombies take their prisoners to be sacrificed. Jolene is to be the first. As the zombies drag her off, Grayvorn fights his way free and begins to lay about himself with his sword. The Doctor and Iris, however, are distracted by the appearance of the holy Relic... which turns out to be a rather nice little gold lamé handbag. One which Iris finds shockingly familiar. Grayvorn grabs the handbag, but is surrounded by zombies and has no choice but to throw it to the Doctor. The zombies turn on the Doctor and Iris, but they and Grayvorn reach the bus safely and escape, unaware until it’s too late that they’ve left Jolene behind. The Zombie King curses those who stole the beloved Relic, and Jolene isn’t much happier; after so many years of searching, she nearly had the Relic in her grasp only to watch as an ignorant barbarian snatched away the power which should be hers. She is already more powerful than anyone suspects, and she unleashes this power upon the Zombie King, blasting him to ashes for his failure. Now, she will lead the zombies, and they will recover the lost Relic.

Unaware of these events, the others assume Jolene to be dead. Iris blames herself, but while the Doctor is distracted by her distress, Grayvorn seizes the Relic, intending to harness its powers and control the afterlife of Artaris. Before the Doctor can stop him, Grayvorn opens the Relic and looks inside, and is transfigured; for a brief moment, he sees the souls of all who will ever live and die on Artaris. He also catches a glimpse of his own future, and at that point he is truly lost. The bus materialises at the convent, where the Mother Superior is waiting; somehow the Doctor senses that only she can handle the Relic safely, but he’s unsure what has led him to this conclusion. Grayvorn, however, refuses to let anyone take the beloved Relic from him -- and he now intends to enslave the Doctor and Iris and force them to operate their time-travelling machines on his behalf.

When the travellers emerge, the Mother Superior rather surprisingly dismisses Jolene’s loss as unimportant. She takes them to the bell tower, where Grayvorn sees all of the mountains and valleys of Artaris laid out before him -- but while he vows to rule it all, the Mother Superior vows to stop him. For all his bluster, Grayvorn is a nobody, a pretend warlord from an obscure realm, and under his rule Artaris would be destroyed. But with the Relic, the Mother Superior will return to power, as she deserves. As the two fight for possession of the Relic, Iris admits to the Doctor that the handbag is hers; she bought it in a bazaar on Hyspero because she liked the look of it, and never bothered to look inside. She must have visited Artaris in the past and carelessly lost her handbag here. The Doctor is infuriated, and tries to intervene between the Mother Superior and Grayvorn. He fails, and the two adversaries stumble over the side of the bell tower as they fight, plummeting to their apparent deaths in the valley below. After all of this effort, the Relic has been lost to the primeval forests once again.

Iris drives the Doctor back to the village of Excelis, where his TARDIS is waiting. He’s angry with her for carelessly altering the destiny of an entire world, even more so when he mentions returning to Frontios and realises from her reaction that she’s visited there as well. Perhaps he does have a certain fondness for her after all, but when she invites him to travel with her for a while, he orders her to stop the bus and let him out. The people of Excelis are hungry for sacrifice, but at this point he prefers the walk -- and if he never sees either Artaris or Iris Wildthyme again, it will be too soon.