The woman in red robes following the travellers runs off to a nearby temple, which houses the Sibylline Sisterhood. She begs audience with the Reverend Mother but it is forbidden. When asked of her message she tells her fellow Sisters that the blue box from their prophecies has finally arrived.
The Doctor and Donna run back to where they landed but discover the TARDIS is gone. A nearby merchant tells them he has sold it to a man named Caecilius, who believed it to be a modern art sculpture.
At his villa, Caecilius examines his new purchase, but his appraisal is cut short when another tremor hits the city. He, his wife Metella, his daughter Evelina and his son Quintus all hurry to protect the decorative items of the house, and once the disturbance has passed resume preparations to welcome their visitor, a man named Lucius.
Evelina and her mother sit beside a large stone vent, which leads down to the fiery underworld of the city. Metella tries to encourage her daughter to access the powers of precognition she appears to have developed recently, and upon gazing into the smoke that rises from the vent she confesses that it makes her feel ill. Her mother leaves her, claiming soon, as the Sisterhood have predicted, she will become a Seer. Evelina once more looks down into the smoke and sees a fiery creature sitting far below.
The Doctor is keen to find the TARDIS and leave but Donna is more concerned with saving the city from Mount Versuvius, which is due to erupt the following day. However, the Time Lord warns his companion that they cannot intervene, the destruction of Pompeii and its people is a fixed point in time, one he cannot allow to change. They argue the matter, but eventually Donna concedes to help find their lost ship.
Meanwhile, the Sisterhood fear that the arrival of the blue box and its inhabitants will spell doom for the city. The Reverend Mother, who speaks from behind a coloured veil, tells them these predictions are untrue, and prophecies the city will soon enjoy a new golden age of prosperity and power. She claims Pompeii will head an empire mightier than Rome itself, and warns that if the two newly arrived travellers threaten this, they must be killed.
Another tremor reverberates around the city and Caecilius and his family once more hurry to protect their possessions, only this time the Doctor and Donna arrive to help. Caecilius assumes they are customers, and reveals he is a purveyor of marble sculptures. The Doctor claims he is an inspector, and warns that the tall blue box in the corner of the room is against regulations. Donna meanwhile tries to convince the family that they should take a holiday as soon as possible, thus saving them from the terror to come. The Doctor prevents her for explaining her intentions to the family and again they argue as to which course of action – intervention or none – is the more moral. They are interrupted when Lucius, a government representative and soothsayer, arrives in the villa.
The family greets him and introduce the Doctor and Donna, who have both taken the pseudonym of Spartacus. The two travellers prepare to leave but stop when they discover the reason for Lucius’ arrival – to view a new marble sculpture he has commissioned Caecilius to produce – one that resembles an electronic circuit.
Evelina, now somewhat unwell, arrives and notes that the Doctor and Donna are sceptical of the prophetic nature of life in Pompeii. Lucius denounces Evelina’s abilities, claiming the prophecies of women are meaningless, only his own and those of men are representative of true precognitive ability.
The Doctor is interested in the vapours from the nearby vent, which Evelina claims aid her in her perceptions. He is interrupted when both of the soothsayers begin to note facts about he travellers they have not revealed, such as their names and places of origin and the fact that the Doctor’s planet has been consumed by fire. Lucius makes two further observations: he claims that Donna has “something” on her back, and then cryptically tells the Doctor that “she is returning”. Evelina notes that the Doctor’s title is a lie; his true name is hidden in the Cascade of Medusa. After claiming he is a lord of time, she faints.
Some time later Metella tends to her daughter. Donna observes and questions why the young woman has a bandage wrapped around her arm. Metella claims it is covering a skin irritation she recently contracted, and takes off the dressing to reveal the skin beneath has turned grey and hard. Donna examines the condition, which she realises is turning the young girl into stone.
Meanwhile, the Doctor examines the vent, which Caecilius claims is a hot spring of Versuvius. The sculptor tells his visitor that the power of the Soothsayers originated after an earthquake that hit the area seventeen years ago, and since then they have been able to predict rainfall and the wealth of local harvests. The Doctor asks if any predictions have been made concerning the next day, but Caecilius claims there are none. The Time Lord then ponders the importance of the vent and it’s counterparts across the city, and realises the Seers have been breathing in the dust of Versuvius.
That night he speaks with Quintus about Lucius, and manages to bribe the young man into taking him to the Seer’s home. They break inside and find a similar vent to the one in Caecilius’ house. The Doctor then examines the other rooms and discovers several more marble sculptures, each resembling a section of a circuit board. Lucius then arrives and tells them the circuits have been made to help create a future dictated to him by the gods.
Donna meanwhile speaks with the revived Evelina about her abilities, and discovers she has been promised to the Sisterhood once her powers have matured. Donna confesses she also has a prophecy, and tells the young girl of the impending disaster. However, she is unaware that the Sisterhood are listening to her words through Evelina, who is now telepathically linked to them. Once Donna has spoken the Sisters at the temple speak with the veiled Reverend Mother, who again claims Donna’s foretelling of destruction is false, and that she must be killed because of this.
Back at Lucius’ villa the Doctor rearranges the marble circuits and explains that it is an energy converter, although its exact purpose evades him. He asks Lucius who designed the full circuit but the Seer doesn’t answer, instead he orders his guards to kill the two intruders. However, before they can strike the Doctor reaches forward and grabs Lucius’ arm, which he reveals is made of stone. He snaps the petrified limb and then runs off with Quintus.
As they flee through the streets Lucius speaks into the volcanic vent and calls for his gods to destroy the Doctor, whom he claims threatens Pompeii. Sure enough, the Doctor and Quintus realise something beneath the ground is following them outside, and are chased by thunderous tremors back to Caecilius’ villa. Upon their arrival the creature following them, the same beast Evelina saw in the smoke earlier, bursts out of the ground and prepares to attack. The Doctor examines the stone monster, which appears to be held together with magma from the volcano.
The Time Lord calls for water to fight the beast with then tries to reason with creature, unaware the Sisterhood have arrived at the villa and taken Donna. Quintus fetches a container of water and douses the creature with it, causing the magma holding it together to cool and extinguish. It’s stone limbs crumble and the beast falls down in pieces.
The Doctor arrives at the Sisterhood’s temple; just as Donna is about to he sacrificed. He frees his companion then speaks with the Sisters about their religion, before being summoned by the Reverend Mother. The senior soothsayer pulls back the veils covering her and reveals her true appearance – humanoid whose skin has turned to stone. Donna recalls seeing Evelina’s arm and realises this is the fate of every Seer. The Doctor confronts the priestess, claiming that there is a creature within her taking over her body. He addresses it and with a rasping voice it speaks through its host, claiming to be a Pyroville. The Sisters begin to chant this name and the Doctor explains to Donna that the creature from the villa that attacked them is also a Pyroville, albeit a fully formed one.
He produces a water pistol from his jacket pocket and aims it at the possessed woman, who explains that the Pyrovilles arrived seventeen years ago; causing the earthquake that shook the city. They embedded themselves in the depths of the mountain and began reaching out to the city’s occupants, using their own powers to open the human’s minds and access their latent psychic ability. The Doctor questions their ability to see through time, which is not within the human capability, and then asks why the approaching eruption has been hidden from the Seers.
His questions are not answered. By now the Sisters have read his mind and determined that the water pistol is not dangerous to them or the Reverend Mother. They advance on him but he escapes with Donna into the underground passageways that link to the vents in Caecilius and Lucius’ houses. The travellers advance through the rocky passageways, heading toward the volcano. Back in the temple the Reverend Mother claims the prophecy she has made about the future of Pompeii must advance. This declaration is heard by all of the Seers across the city, including Lucius, who rallies his group of guards and prepares to leave for the mountain.
As the sun rises on the day of the eruption, Metella worries that the creature they killed the previous night was a messenger from the gods, and in destroying it they have brought great misfortune upon themselves. Evelina enters and claims that she foresees a great decision being made, one that will have terrible consequences.
Still underground, Donna questions the Doctor’s decision to save the Earth in her own time but still let Pompeii be destroyed. He claims that he sees time in a different way to her, he can see what must be changed and what must remain constant, that is his burden. Donna continues to try and persuade him to save the city, disturbed that he is prepared to let twenty thousand people die.
On the hillside surrounding the mountain, Lucius and his men advance with the marble circuits. They arrive at an opening in the volcano and are greeted by a gigantic Pyroville. Inside, the Doctor and Donna find themselves emerging into the very heart of Versurius, which is now overrun with Pyrovilles. The Doctor examines the remains of the creature’s ship, but is interrupted when Lucius arrives. He calls for the stone warriors to destroy the two intruders but the Doctor again uses his water pistol to fend them off.
He asks what the creatures plan to do when the Seers are fully developed into Pyrovilles, and Lucius explains that they intend to create an empire spanning the whole of the Earth. Donna asks why they cannot rebuild their ship and go home instead, and Lucius again answers, claiming their home planet has disappeared. The Doctor is confused by this but nevertheless vows he must stop the creatures for the good of the planet. He and Donna conceal themselves in a nearby pod; in which are similar marble circuits and controls to those Lucius has been collecting.
The Doctor realises that the energy converter will take the lava from Versuvius and use it to convert the rest of the human race into Pyrovilles. The Soothsayers cannot foresee the volcano erupting because with its power gone it never will. He realises that by destroying the converter and stopping the Pyrovilles he will trigger the eruption – the destruction of Pompeii is not just part of history, it happens because of him.
He prepares to invert the converter but wavers, not sure of what to do. At last Donna understands his decision – save one city or the entire world – and together they press the lever that will cause the mountain to explode. Lucius and the other Seers cry out, realising that the future as they see it is changing.
The converter pours energy back into the volcano and it erupts, sending a great column of smoke into the atmosphere and huge clouds of ash onto the city. The force of the explosion throws the Doctor and Donna’s pod out of the mountain, and it lands on the hillside nearby. As smoke blocks out the sun and the surrounding area is plunged into darkness, the two travellers run back to the city, where the inhabitants have descended into panic. Ash falls from the sky like snow and back at the temple, the Sisterhood realise the end is nigh.
The Doctor is adamant he and Donna must return to the TARDIS and leave, but his companion has other ideas. She runs through the streets screaming for the people to head toward the hills so they will be safe, but her cries fall on deaf ears. Those still left in the city run for the beach, which is soon to be buried in ash.
Donna and the Doctor return to the house of Caecilius, where the family is cowering in fear of the mounting eruption. Caecilius calls out to the Doctor to save them but the Time Lord does not respond. He returns to the TARDIS and begins to take off, Donna reluctantly following on behind. As the ship leaves she orders him to go back to Pompeii and save those in danger, claiming it is not fair that they should die when the volcano’s eruption was pre-empted.
She tries to convince him, claiming his own planet burned like Pompeii and he cannot allow such a thing to happen again. Once more he tells her he cannot intervene, he was forced to let his planet die and the laws of time do not permit him to go back and save it, the same rules must apply to Pompeii. Tearfully, Donna begs him to save just one person, to prove he can make a change. Reluctantly he returns to the villa and tells Caecilius and his family to enter the ship.
Some time later, as the city is buried under the volcano’s ash, the Doctor, Donna, Caecilius and his family look on from the hillside. The Doctor tells them that the city will never be forgotten, for it will be uncovered in a thousand years time. Donna asks Evelina if she can still see into the future, but she claims the visions are gone. The Doctor explains that the explosion caused a rift in time and space to appear for a few moments, which echoed back to the Pyrovilles and allowed them to see into the future, an ability they passed on to the Seers of the city. Before they leave the Doctor tells the family not to mention his or Donna’s presence in the city at the time of the eruption, and at last he and his friend depart.
Inside the ship Donna thanks the Doctor for saving the family, and he in turn thanks her for directing him, admitting at last that as she once told him, he needs someone to save him from himself.
Six months later, Caecilius and his family are living in Rome. Evelina now leads the life of a normal teenage girl and Quintus is studying to become a Doctor. As he prepares to leave the villa and continue his studies he stops to show gratitude to the household gods. He kneels down and thanks the stone carving that depicts the two saviours – the Doctor and Donna.