Pompeii, 1980 AD. An earthquake has damaged one of the greatest archaeological treasures of the Roman world, and uncovered something inexplicable in the process. Captain Muriel Frost of UNIT is called to the scene, and once she sees the object she orders the confused Professor Scarlini to remain silent about his discovery. Nobody else must know that an English police box was buried beneath volcanic ash in the year 79 AD...

The TARDIS has arrived in the Roman Empire, but for some reason it refuses to tell the Doctor just where or when they are. When the Doctor and Mel emerge they encounter a cowering slave, Tibernus, who has witnessed their materialisation and believes them to be messengers of Isis. The Doctor chooses not to complicate matters by correcting him, and simply asks what day it is. When he learns that it is the day of the Vulcanalia -- the tenth before the calens of September, or 23 August -- the suddenly sombre Doctor allows Tibernus to depart without bothering to ask the year, for he's afraid he already knows. It is 79 AD, they are in the city of Pompeii, and just beyond the buildings he and Mel can see the slopes of Mount Vesuvius. The Doctor, seeming preoccupied, tells Mel that her clothing may attract unwanted attention, and that it could be dangerous to meddle with history; it would be best if they were to leave straight away. But it must be her choice. As the Doctor seems unwilling to tell her what's bothering him, she chooses to have a quick look around. Everything is proceeding just as he'd expected... and tomorrow, at mid-day, Vesuvius will erupt, destroying Pompeii and killing thousands of innocent people.

In the Via dell'Abbondanza, the Street of Plenty, Mel and the Doctor see graffiti mocking the cowardice of the gladiator Polycarbus, and Mel is momentarily separated from the Doctor in the crowds when she notices a young man staring at her. A homeless old man then begs her for spare coins, and is shocked and enraged when she claims to have none; how dare someone with such clean clothes and skin claim poverty? Realising that she has offended him and is drawing attention to herself, Mel tries to cover up by repeating the Doctor's claim to be a messenger of Isis. The Doctor shows up and ushers her away, telling her not to spread that rumour any further. But the damage has been done; the young man who was watching Mel is the decurione Popidius Celsinus, a member of the municipal council, and when he questions the beggar, the still-offended older man repeats Mel's claims. And is it not odd that Celsinus of all people should not have known that messengers of Isis had arrived in the city? Meanwhile, Tibernus' mistress Eumachia threatens to flog him for his tardiness, and the terrified slave claims to have been detained by messengers of Isis. Eumachia isn't pleased; she follows the one true faith and has no love for the foreign goddess. Without realising it, the Doctor and Mel have just made a powerful enemy...

The streets of Pompeii are crowded with those who are visiting the city for the Vulcanalia and the Festival of the Divine Augustus. The Doctor and Mel pause to rest at the inn of Valeria Hedone, where they see rotting fish laid out as an offering to Vulcan. The Doctor finds himself short of cash, and thus approaches a group of gambling gladiators and offers his "maidservant" Mel as collateral. The gladiators' arrogant leader, Murranus, accepts the Doctor's wager, and the irritated Mel has no choice but to remain silent as the Doctor plays. He wins the first round, but Murranus does not take losing well and demands that the Doctor give him the chance to win back what he has lost. The Doctor has little choice, but before resuming play he gives some of his winnings to Valeria and asks her to buy appropriate clothing for Mel. Valeria delegates this task to the slave Aglae, and Mel, who wants to do some shopping herself, accompanies her. Aglae is shy but gladly talks with Mel, who is worried about the Doctor's downcast behaviour and is happy to have someone to talk to. However, when they arrive at Aglae's home and place of business, the Lupinae, Mel is startled by the graphic carvings above the entrance and is taken aback to realise that her new friend works in a brothel. When Popidius Celsinus reappears and heads directly towards Mel, she darts into the brothel with Aglae to avoid him before realising that this will probably give him entirely the wrong impression. While waiting for him to tire and leave, Mel changes into a stola, but chooses to keep her own clothing beneath. When she and Aglae emerge, however, they see the citizens staring in awe at a column of smoke rising from Mount Vesuvius. To Mel's horror, the earth begins to shake...

The Doctor has allowed Murranus to win back ten sestersis, but this isn't enough for Murranus, who will not let the Doctor go until he has fully recovered his losses. The Doctor has no more coins to wager, however, having spent the remainder on Mel, and in any case he's had enough of Murranus; although it does take skill to throw a loaded die properly, it's not the kind of skill that impresses the Doctor. Enraged by the accusation, Murranus attacks the Doctor and tries to throttle the life out of him, but the earthquake interrupts him and Valeria ushers the Doctor out of her inn to avoid trouble. Murranus loses him in the crowds outside; however, he neither forgets nor forgives the Doctor's all too accurate slur on his reputation, and vows to avenge himself...

While still in his fifth body, the Doctor visited Earth in the early 1980s, to be told by UNIT that the TARDIS had been found buried in Pompeii. He has always hoped that there might be an innocent explanation, but when he arrived in the city the day before the eruption he knew what was fated to happen. So did the TARDIS, which is why it didn't tell him where they were. Mel still doesn't understand why the Doctor didn't just leave, and he tries to explain the concept of a paradox loop to her; he can't avoid a situation he's learned of in the future, or else the situation won't happen, he won't learn of it and thus he won't avoid it. Mel, however, simply can't accept that something which hasn't happened yet has already happened. As the Doctor has given up all hope, Mel sets off to save the situation herself. The Doctor remains at Valeria's inn, where even the recent graffiti mocking Murranus' humiliating defeat can't shake his morose mood. Valeria suggests that the Doctor punish his strong-willed maidservant for her own good, and urges him to make a sacrifice to Isis for counsel in his time of trouble. Valeria herself would gladly sacrifice her own life for her goddess. But the Doctor knows it isn't just his life that he's being asked to sacrifice...

Eumachia attempts to enlist Celsinus in her plot to expose the newcomers as frauds. Celsinus' father paid for the reconstruction of Isis' temple following the earthquakes of seventeen years past, thus buying his son a seat on the municipal council. But now that messengers of Isis has arrived in the city, and have visited neither him nor the rebuilt temple, rumours are beginning to spread. What will his position be worth if it becomes known that his goddess has spurned him? Leaving Popidius to consider these uncomfortable questions, Eumachia purchases some time with Aglae and questinos her about Mel. Dissatisfied with the answers, she tries to beat some sense out of Aglae, but Mel finds her doing so and stops her, urging Aglae not to put up with this treatment. Aglae, however, is distressed by Mel's attitude and returns to her mistress, insisting that Mel's hubris will bring the gods' wrath down upon them both. Mel, frustrated and angry, is in no mood to treat Eumachia with the respect due to a priestess of the true religion and the owner of Pompeii's wool market. As Mel storms away, Eumachia is secretly delighted; bringing down this arrogant, blasphemous stranger will be almost too easy.

Later, Aglae finds Mel trying to help clear the rubble away from the corner where the TARDIS is buried. Aglae's mistress had sent her out into the streets to seek custom, but, regretting her harsh words, Aglae has sought out Mel instead. Mel realises that she'll never get the rubble cleared away in time, and, sensing that her friend is troubled, Aglae offers to take her to one who can help. Mel soon realises, however, that Aglae is referring to her goddess. The priests of Isis are preparing to sacrifice a goat, and although sickened by the brutality, Mel knows that even were she to protest it would do no good -- and it might be the very act which traps her in Pompeii for the rest of her life. As she leaves the temple, she runs into Celsinus, who confesses that he has been following her; as a decurione he is naturally interested in visitors to his city. He invites her and the Doctor to dinner, and although Mel is at first reluctant -- particularly when it becomes clear that Aglae is not included in the invitation -- she realises that Celsinus may have authority over street works, and that he will be able to get the rubble cleared away more quickly. She therefore accepts.

Mel and Celsinus collect the Doctor from Valeria's inn, which is fortunate for the Doctor, as Murranus arrives soon afterwards to drown his sorrows. Already the people are talking about his defeat at the hands of a deceitful stranger, and Murranus fears the effect this will have in the arena. When he was sent to the amphitheatre as punishment for his crimes, the aediles believed they had sentenced him to death. He has only survived this long due to his skill as a warrior, but even he cannot win every battle, and when he loses he depends upon the people to spare his life. The loss of his reputation may very well mean his death, and the only way to put matters right and ensure his safety is to avenge himself against the Doctor...

It's dark and it's pouring rain, and Murranus has fallen asleep in Valeria's inn. Valeria, who needs her sleep before the busy Vulcanalia tomorrow, leaves him to rest where he lies. Outside, the Doctor finds Aglae and asks her to help him rescue Mel. He's overlooked something terribly obvious, and time is working against him...

Mel perks up when a visitor arrives at the gaol, but she's less pleased when it turns out to be Celsinus, who can't get her out of his mind. She admits that she lied about being a messenger from Isis, but insists that she did not steal Eumachia's jewellery. Celsinus senses no evil in her, and when he thinks about it he realises he was with her all the time she was in Eumachia's home. He realises that Eumachia manipulated him into betraying Mel and discrediting his own goddess, and promises to speak to the aediles; if all goes well, he will have her free of the cells within days. Stunned, Mel asks him to leave her, and although puzzled and hurt, he does so, promising to pray to Isis for her sake. As he leaves the cells, he finds the guard asleep and the Doctor searching for the keys. Realising that Celsinus is tired of the responsibility of being a decurione, and that he has the keys, the Doctor hypnotises Celsinus as well, takes the keys and frees Mel. Aglae has stolen her mistress' horse and wagon and is waiting to take Mel out of the city. The Doctor will remain to find the TARDIS, having finally realised that the building which collapsed was excavated long before 1980. The TARDIS must have been moved somewhere else, and the Doctor intends to find out where, and by whom.

This time, Mel concedes that she has little choice and agrees to leave the Doctor. Aglae takes her to the Herculaneum gate, and tells the legionary on guard that she's on an errand to fetch supplies for Valeria. The suspicious legionary searches the cart and finds Mel, but much to her own surprise, Aglae knocks him out with the arm from a marble statue which was damaged by the earlier earthquake. Horrified by what she has done, she flees on foot with Mel, convinced that she will be sent to the lions for her crime. She and Mel hide in the necropolis for the night, and Aglae struggles to understand what possessed her to defy her mistress and the will of the gods. Mel assures her that she has done the right thing, and that soon, it won't matter what the people she's left behind will think of her. As dawn breaks, Mel opens the tomb to check whether the coast is clear; unfortunately, it isn't. The legionary they assaulted is waiting for them outside...

The Doctor needs somewhere quiet to sit and think and thus heads for Valeria's inn, unaware that Murranus has slept the entire night there. Murranus hears him at the door, asking to be let in, but unfortunately for the gladiator he is too hung over to restore his honour. Valeria wants no trouble, but it's here nonetheless, and unless she helps to deliver the Doctor into Murranus' hands then Murranus will kill her as well. Valeria thus reluctantly hides Murranus in her back room and plies the Doctor with drink when he enters. He needs to keep a clear head to consider the patterns of history, but there's too much for him to consider and he can't even keep the politics of Pompeii straight. He does learn that Eumachia's bitterness towards Isis stems from her own position as a high priestess to the triad of Jupiter, Juno and Minerva; the worship of Isis threatens her own power. The Doctor eventually concedes to Valeria's offer and takes a drink, but realises too late that it's been drugged. The last thing he sees as he passes out is Murranus returning and gloating over his downfall. Valeria, ashamed of herself, bars Murranus from her inn, but the damage has been done; Murranus' honour will soon be restored...

Once again Mel is in the cells, and this time it's even worse; she's managed to get her friend Aglae locked up as well, and there will be no rescue from the Doctor, who believes them safely over the hills and far away by now. With only hours left to her, Mel has no choice but to send a message to Popidius Celsinus, asking to speak privately with him outside. This time she tells him the whole truth, which he finds difficult to believe. How can she claim that the gods will destroy Pompeii, when the oracles have made no prophecy of the doom to come? Mel begs him to give her the benefit of the doubt and help her to find her lost property, but when he describes it just as Eumachia had described it to him -- right down to the English writing on the sides -- Mel realises that Eumachia must have seen it herself to have described it so perfectly. Once again she has tried to discredit Isis by separating the goddess' messengers from their temple. Mel is triumphant -- for a moment, until the earth quakes again. Again, the tremor seems to pass quickly... only to be replaced by an unnatural calm. The water has stopped flowing from the fountains, and seagulls have vanished from the sky, and Mel and Celsinus see a caged bird fluttering frantically against its bars. Mel's revelation has come too late; Vesuvius is going to erupt at any moment...

Mel urges Celsinus to let her go to Eumachia's house, while he does what he can to save himself -- and Aglae, who will die if she's still locked in her cell when Pompeii falls. Celsinus makes his choice and agrees to do as she asks. Meanwhile, Valeria can't bear the guilt of betraying the Doctor and goes to the ampitheatre to stop Murranus, but Murranus is perfectly willing to go through her to get to the Doctor. The Doctor doesn't wish to fight Murranus, but he takes up arms to protect Valeria; however, although he manages to tangle Murranus in his own net, the gladiators are still blocking the exits and Valeria is wounded when she and the Doctor try to flee. The Doctor lags behind to help her and is surrounded, but as Murranus approaches for the kill, the Doctor waits for the distraction he knows is coming...

And Mount Vesuvius erupts.

The citizens of Pompeii panic as the sky fills with smoke, fire and ash. Mel is only a block from Eumachia's home when a legionary finds her and escorts her to shelter. As it happens this is the same legionary whom she and Aglae assaulted at the Herculaneum Gate, but even he can see that they're beyond recriminations now. Rocks and ash are falling from the sky and Mel can't even see for more than a block, let alone cross a city. But after a moment's despair she pulls herself together, determined to carry on, even though she can't even get into the TARDIS without the Doctor. The legionary refuses to leave shelter, and Mel, although grateful for his well-meant attempt to help her, must abandon him to his fate.

Valeria and the Doctor flee from the ampitheatre in the confusion, and as the sky turns dark, Valeria marvels at the Doctor's power; surely he must be a messenger of the gods indeed, that an attack upon him should bring such wrath down upon the city. The Doctor, however, insists that this is a natural phenomenon; Vesuvius may have been dormant for so long that vegetation has covered its slopes, but it's still a volcano. The gods have not decreed their fate, and they have a slim chance of survival if they can get out of the city. Six feet of ash will rain down on Pompeii for the next twelve hours, at which point a river of boiling lava will flood down the slopes of the volcano and engulf the city, killing all those who had tried to shelter indoors. Pompeii will be wiped from the face of the Earth... but it will rise again, one day. In the meantime, Valeria must forget her home and possessions, and concentrate on saving her life -- and the Doctor's, as he needs her help to navigate the city streets and escape. He promised a friend that he wouldn't die here, and he hopes she's far enough away to return the favour...

In fact, however, Mel is still in Pompeii, and although she makes it to Eumachia's house, the priestess blames her for bringing this destruction down upon the city with her blasphemy and lies. Even when flaming rocks crash down through her own roof, Eumachia insists that this is because Mel is there. Realising that Eumachia will never tell her where the TARDIS is hidden, Mel tries to use reason. Eumachia would never hide the blasphemous temple of Isis within her own home, and the woolen market is too public -- but her family tomb in the necropolis would make the perfect hiding place. As Mel leaves, however, she runs into the slave Tibernus, who still believes her to be a messenger of Isis and who reluctantly admits that he and his fellow slaves hid her temple in the tomb of another family, so that if it were found nobody would know that Eumachia was responsible. Unfortunately, he cannot ready, and doesn't know whose tomb it was hidden in; only that it was located near the Nucerian Gate. Mel urges him to lead her to it, but Eumachia, confident that her gods will protect her, has ordered her slaves to remain in her house with her, and Tibernus is too frightened of her to disobey. Mel must leave, knowing that Tibernus can't be saved; his mistress' obsession will get herself and her slaves killed. Elsewhere, Murranus and the gladiators are guilty of the same arrogance, having retreated to their barracks in the confident belief that the gods will not desert those who honour them with their skill and courage.

The Doctor and Valeria have almost reached the city gate when they run into Popidius Celsinus and Aglae, who are also trying to reach safety in the crowds. The Doctor is appalled to see Aglae, and is horrified to learn that Mel is still in Pompeii, searching for the TARDIS. He passes Valeria over to the care of Celsinus and Aglae, telling them to keep going; life will go on, and even Pompeii will one day be rebuilt on a different site. Wishing them good luck, the Doctor turns back to look for Mel. The others struggle on, but in the confusion, Celsinus and Aglae lose Valeria amongst the crowds. Nevertheless, they press onwards, to the city gate and beyond, to the fields and the possibility of safety. Despite their very different positions in society, Celsinus does not abandon Aglae; he promised Mel that he would keep her safe, and he will keep that promise, whatever it takes.