These are the last days of Venice. The grand marble palaces are sinking into the lagoon and the crowd is partying as if there's no tomorrow, which there isn't. The ruler of the city, Duke Orsino, does not wish his city to be saved; he is mired in self-pity and melancholy, and can think of nothing else but the loss of his love Estella. His curator, Churchwell, sorrows to see Orsino dwelling on the past, but what else do they have left? Tomorrow morning the city will be gone forever, but all Orsino wants is to see Estella one last time, to beg forgiveness for gambling away what he could never replace -- her love. It is little wonder that she cursed him and this city to destruction after such a betrayal. Churchwell is concerned with saving the paintings and other artistic treasures that remain in Orsino's private collection, but the duke has little taste for art and less interest in saving what's left. The last boats have gone, the lagoon is sealed, and all who remain will die when the city sinks beneath the waves. Orsino knows his people are waiting for him, and as he departs for the ballroom, still praying for Estella's miraculous return, Churchwell returns to his gallery to see his beloved paintings one last time.

Charley has never been to Venice, which has always seemed a gloomy place to her; but on the other hand, she's always wanted to travel. Her father was never keen on her striking out alone, and she can't wait to return home and shock her family with tales of her travels. The Doctor quietly changes the subject back to Venice, a charming and sinister city, desolate and ruined by day but glittering, starlit and magical by night; but although he's aimed for the Renaissance, the TARDIS materialises in the 23rd century, in the city's last days. Most of its inhabitants have evacuated, but the Doctor and Charley meet one of the few who have remained; the aptly-named Eleanor Lavish, a grand old dame who is far too elderly to go running away from her home. Soon, Venice will be beneath the lagoon once more, and Ms Lavish will find peace at last as the fish take up residence in the grand ballrooms. Charley is disgusted with the Doctor, who was supposed to take her somewhere glamorous and exciting, but instead brought her to a dying city full of fatalistic revellers and the suicidally depressed. The Doctor insists that the fall of Venice was always on the cards, a phrase which seems to amuse Ms Lavish immensely; Venice's doom comes not from the natural process of decay, but the curse. That's all she's going to say for now, but she feels sure that they will meet again as events move to a climax. As Ms Lavish stalks off, the Doctor and Charley head out to find the last great party at the end of the world, which is no more than Ms Lavish had expected of them. Everyone here seems to believe that Venice will somehow survive, but Ms Lavish knows that there's no such thing as magic. There will be no miracle. Venice's fate is sealed...

As the web-footed gondolier Pietro punts the Doctor and Charley along the canals, the distant sound of cheers and fireworks can be heard. Charley decides to enjoy herself after all, but the Doctor has caught a chill of melancholy. Pietro realises that his passengers are genuinely respectful, unlike the other drunken and foolish revellers, whose only interest is in their own entertainment. Pietro and his people are looking forward to the city's death; then, it will belong to them. They arrive at their destination, and Charley, who was expecting a party, is surprised and irritated to find that the Doctor has instead taken her to an art gallery where surrealistic paintings by Magritte and Ernst hang on display. Charley is far more interested in Pietro's resentful attitude, and she ditches the Doctor to speak with Pietro. Gondoliers are not permitted to consort with visitors, and Pietro thus offers to take Charley to a hidden place where his people rest. On the way, they hide from a group of hooded figures who seem to be searching the streets for something; driven by their leader, the High Priest Vincenzo, the rather unpleasant cult of Estella has ventured out into the growing twilight of Venice's final evening. With only hours left, they continue to search for the last thing they require for their ceremonies to be complete...

The Doctor is no longer as good at bluffing his way into places as he used to be, but he knows enough about Churchwell -- and is respectful enough of the art -- to convince Churchwell to give him a guided tour. The Doctor is particularly fascinated by the unattributed surrealist paintings, and is appalled to learn that the Duke is willing to let them all sink into the sea. But according to Churchwell, all the Duke ever does is pine for his lost love Estella. There is nothing remaining of her but the curse; there are not even any paintings of her, although the cultists who worship her refuse to believe this and Churchwell is hounded day and night by fanatics who believe that he's hoarding her portait somewhere in his gallery. As far as he is concerned, Estella died a hundred years ago; the Duke is only alive now because his life was prolonged by the curse, so he could live a hundred years and regret his betrayal of Estella for every minute of them. Churchwell is disturbed by the Doctor's fascination with this story, and by his claim that a painting of well-dressed foxes in a volcanic landscape is not surrealist at all; but as he tries to usher the Doctor out of his gallery, the Doctor brings him up short by claiming to have a ship which he can use to get Churchwell's beloved treasures out of the city before its destruction. However, just as he catches Churchwell's attention, he realises for the first time that Charley is nowhere to be seen. He thus rushes off in search of her, with the suddenly desperate Churchwell at his heels.

Pietro takes Charley to a ramshackle dwelling where the gondoliers rest and plot against the complacent upper classes who treat them as nothing more than dull-witted transport. They've been oppressed by the self-indulgent Orsino for a hundred years, and are looking forward to the change; but there's a problem. Orsino has been searching the city for the past century, hoping to find his beloved Estella's remains, bring her back to life, and lift the curse; but this is the last thing the gondoliers want, for when the city falls, the aristocracy will fall with it and the gondoliers will take back their home. Charley sympathises, but she doesn't see what she can do to help until Pietro explains; they need to prevent the Duke from finding Estella before the city falls, and they intend to do this by presenting him with Estella herself, risen from the dead. As Charley tries to take her polite leave, the gondoliers present her, drawing weapons to keep her where she is. She's going to play the part of Estella for Orsino whether she wants to or not...

Imprisonment in the underground lair of an evil cult is just part of an ordinary day out for the Doctor, but Churchwell isn't taking it very well at all. For years he's hidden away from the cultists, knowing that they'll never believe his claim that no portrait of Estella exists; and now the Doctor has delivered him right into their hands. They're below sea level here, and the walls are creaking and crumbling -- and the Doctor finds that he can pull down the masonry of one wall to reveal a way out of their cell. Churchwell reluctantly clambers out into a damp corridor, in which they can hear the distant echoes of chanting as the cultists practice their necromantic ceremonies. It's the last night of Venice, they're surrounded by danger and dark secrets, and much to Churchwell's horror, the Doctor is in his element; for there's corruption here, and he's the man to sort it all out. Churchwell may think him mad or hopelessly naive, but it's a viewpoint which has stood him well so far.

Charley wakes, disoriented, to find herself back in a gondola, dressed in an elaborate frock, with voices whispering in her mind. She's been drugged and dressed in Estella's wedding gown, which the gondoliers fished from the canals generations ago. Pietro will lead her into the ballroom, and the drug will do the rest, filling her mind with the words she must say. Pietro has nothing but contempt for the waiting revellers, who dance, feast and drink as the water rises beneath them, with no thought for the gondoldiers who have suffered under a century's rule by a weak, self-indulgent duke. Charley may have shown his people sympathy, but he can't afford to treat her as a friend, or to trust her. Charley disagrees, but the drug has hold of her body now, and she has little choice but to relax into the flow of things, and become, for the moment, Estella reincarnate.

Churchwell wanted to spend the evening alone and die with dignity, but instead he's crawling for his life through a damp corridor which turns out to lead straight to the cult's most holy sanctum. Here, they find a clock ticking backwards towards dawn, and a large golden casket, undoubtedly the coffin of Estella herself. To Churchwell's horror, the Doctor promptly levers the coffin open to have a look for himself... and finds the coffin empty, but for dust and a few ancient jewels. The Doctor takes the jewels for safekeeping, and closes the coffin again -- mere moments before Vincenzo and his cultists burst in to find them trespassing. Rather than put them to death immediately, Vincenzo gives them a chance to save themselves; they must infiltrate Orsino's palace and steal the portrait of Estella. The weary Churchwell insists once more that there is no such portrait, but the Doctor innocently claims that Churchwell told him it was hanging in the ducal apartments. This is just what Vincenzo wants to hear, and he triumphantly orders his guards to take the Doctor and Churchwell to the palace.

The music in Orsino's grand ballroom judders to a halt as Pietro and Charley enter. Orsino is so outraged to see a gondolier in his palace that at first he fails to notice what Charley is wearing -- but then Charley speaks the words in her head, claiming to be Estella, reborn and returned to forgive him for gambling her away. The stunned Orsino orders his gaping guests to return to their entertainments, and loses his temper when Ms Lavish prefers to remain where she is, watching. The legends say that Estella will be brought back to the Duke by the lowest of the low, and there are few lower than Pietro; but how can Orsino allow himself to believe in what he's wanted for so long? He may look as young as ever, but he's a tired old man inside, and probably wouldn't recognise Estella if she were standing right before him. Nevertheless, he dare not risk rejecting her; the revellers are here to see the curse fulfilled, and if the dawn brings only anticlimax they'll be profoundly disappointed. Charley realises that Orsino fears his people's anger more than he desires to see Estella again; one hundred years later, he is still just as selfish as he ever was. Pietro and Ms Lavish escort Charley to Estella's apartments to prepare for the dawn, and find the rooms thick with a century of dust. Pietro and Ms Lavish find themselves oddly disappointed; even after a hundred years of pining and self-indulgency, somehow they still believed that the Duke would save them all. But only the true Estella can do that, and she's not going to put in an appearance tonight, whatever the cultists may think. Charley is furious; thanks to Pietro's machinations, she's now expected to die alongside the Duke at dawn, while everybody looks to her to save them all...

Ms Lavish decides to return to the party, and she urges Pietro to come along; they may as well spend their last night on Earth enjoying themselves. But Pietro smugly informs her that his people will not die. "Fish people" is not just an insult; the gondoliers have evolved into amphibians, and when Venice sinks beneath the waves they'll make the city their own. Charley has no intention of sticking around to see that; the effect of the potion has worn off, and she points out that the gondoliers don't need a distraction to prevent the Duke from saving the city, as he obviously has no intention of doing so. Pietro, who does in fact regret involving her, agrees to help her slip out.

The gondoliers' attack took place very close to the palace, and the Doctor and Churchwell are thus rescued by the duke's guards. Unfortunately, so is Vincenzo, and even the deaths of his priests haven't dampened his determination. The Doctor leads them into the palace to confront Orsino, creating quite a stir and interrupting Charley's escape attempt. The Duke takes little interest in the new arrivals until Vincenzo identifies himself as the high priest of the cult who dares to worship the duke's beloved Estella; however, before Orsino can have him arrested, Vincenzo reveals that he has Estella's remains and is willing to bargain them in exchange for the legendary portrait. The Duke can't believe this, not even when Vincenzo claims that it was Estella herself who founded her own cult and entrusted her remains to Vincenzo's grandfather. Or so the story has been passed down to him. The duke is enraged to think that the cultists who claim to worship Estella would keep her remains hidden from the man who loved her most; but what else can he expect, after his betrayal drove her to take her own live? In all the hurly-burly, they pay little attention to the Doctor's attempt to change the subject to the art in Churchwell's gallery -- allegedly surrealistic paintings which the Doctor believes depict actual places, far from Earth.

Charley, Pietro and Ms Lavish approach, throwing the matter into even more confusion; Vincenzo is enraged by Charley's blasphemous claim to be Estella herself, and with the Duke's apparent decision to remarry at dawn. Orsino has had enough; he takes a knife and cuts Vincenzo, making him bleed but letting him live so he can guide the duke to Estella's remains. Ms Lavish is disgusted with them all; what's the point of fighting amongst themselves, while the city is sinking into the sea beneath their feet? The Doctor is more concerned with Charley's strange behaviour; what has she been up to while he's been away? Orsino ignores them all and prepares to storm off to the cult's lair, but Ms Lavish warns that this will only cause worse bloodshed. Amused by her presumption, Orsino announces to his people that he's taking his leave of them; until he returns, Ms Lavish is to take the throne. And then he storms off, accompanied by the Doctor, Charley, Vincenzo, and Churchwell -- off to see the world turn upside-down before it ends completely.

As the ducal barge pulls away from the palace, the buildings begin to fall into the sea around them, while the duke laughs like a madman. The Doctor learns what Charley's been up to, and she assures him that she's no longer hypnotised. But Pietro has a lot to answer for, and while he personally may be embarrassed about what he's done, his people still have a lot of anger; even now, they're shadowing the ducal barge, awaiting their chance to attack. This isn't quite what Charley expected when she first joined the Doctor on his travels. As they approach the cult's lair, the Doctor tries to talk sense into them all; why not work together to find a real solution? His ship is nearby, and they could scour the city for survivors and get them all to safety. Nobody listens, and he knows that things are only going to get worse; if they try to open the casket, they really aren't going to like what they see...

Back at the palace, the duke's wine cellars empty as the party continues. Ms Lavish watches the dancing fools with amusement, pity and contempt, but allows them to enjoy themselves while they still can. She was hoping for a quiet end to it all, hoping to sink gracefully beneath the waves; and yet here she is in the thick of things...

It's time for the party to end, but nobody listens to Ms Lavish's pleas for sanity. If they really expected Estella to return and save them all, they've got a nasty shock coming. Venice begins to sink, and as the panic-stricken revellers flee for their lives, Ms Lavish sighs and orders the band to strike up. After all, there's nowhere to run to...

Even as the catacombs crumble about them, Vincenzo and Orsino are at each other's throats over the loss of Estella's remains. The Doctor admits that he took a peek earlier, and already knew that they were gone -- but he's taken aback when Orsino suddenly becomes desperate to find her missing jewels. Vincenzo concludes that Estella must have risen already, and that she walks amongst them even now. Then the gondoliers attack, and as they and the cultists do battle, Orsino flees back to the barge, followed by the other main players. The city is dying, masonry crumbling and buildings sinking; somewhere, all of Churchwell's beloved art is being destroyed. Only Estella can save them now, and once again Vincenzo demands the portrait, the only means by which they can summon her. To Churchwell's shock, Orsino admits that there is a portait, the only memento she allowed him to keep. Charley feels that it's too late and that they should make for the open sea, but Orsino refuses to take orders from a crass impostor such as her. Charley, just as enraged, reminds him that he was willing enough to go along with the charade to save himself; it's his own self-indulgence, misery and greed which brought this fate upon the city. Nevertheless, they return to the ducal palace as he wishes -- and to their surprise, it's still standing. The Doctor realises that there's some other force at work here...

The palace is deserted but for Ms Lavish and the musicians, and the musicians stop playing and run for it even as the ducal party arrives. It's time for Orsino to fetch the portait, but Ms Lavish has had enough of the charade, and she calls Churchwell up to the throne to examine her brooch. The portrait which Estella gave Orsino was small enough for her to palm into his hand the day she left him; so small that he didn't realise what he had until she was gone. It was small enough to be worn as a cameo... and to Churchwell's astonishment, it's the cameo which Ms Lavish has been wearing all along. It's a portrait of Ms Lavish in her youth -- a portrait of Estella... Vincenzo bows down before her, much to her disgust; she hasn't risen from the dead, because she was never dead to begin with. She regards the mythology which has sprung up in her absence as ridiculous, a ritual to clutter up a simple mystery. And Orsino is no better; he has squandered his extra life with moping and self-pity, just as he squandered everything else she ever gave him. The city deserves to be given over to Pietro's people.

The Doctor has had enough. There's no such thing as prophecy; there are certain true events which cannot be avoided, as he knows only too terribly well, but myths and destinies are just hogwash. All that is happening now has been caused by the will of two people, and it's up to those two people to stop it. But Estella refuses; she damned the city quite thoroughly, and she couldn't reverse its fall even if she wanted to. The Doctor then reveals the source of her power -- the jewels which he took from her "tomb", and which she uses to focus her willpower. The curse is not magical, but the effect of alien technology -- the technology of Estella's own people. The Doctor guessed the truth when he realised that some of the "surreal" paintings in the gallery were in fact portraits of worlds which Earth has never encountered. Estella admits that she's alien, but still refuses to take the blame for what is happening. All she did was fall in love with a foolish, indolent man, who made her a part of his world and then cast her aside without a thought when it was too late for her to return to her home. He may be sorry for what he did, truly sorry, but she does not accept the apology.

Vincenzo's prayers have been wasted, and Orsino finally realises just how badly he has damaged Estella. The woman he loved has been replaced with this bitter, twisted old hag; or perhaps she is just tired. Very old, and very tired. But even as she admits this, she reveals that the curse cannot be lifted; magic has a balance, and for one life to be prolonged, another must decay. Venice's collapse has been accelerated because Orsino and Estella's lives have been prolonged. Orsino, accepting full responsibility for his crime at last, tells the Doctor to give him the jewels, and, realising that Orsino truly intends to make amends, the Doctor does so. Despite Estella's protests, the duke dons her necklace, and channels his thoughts out into the city. He's lived too long already, and he calls to her to join him in the flames and grow old with him. To her surprise, she realises that she still loves him; she always has. There is no more need for bitterness, and Orsino can't save the city by himself. Despite Vincenzo's protests, Estella joins the duke in the flames as the necklace's power strips away the last of his extended life. When the light dies away, only two charred bodies remain. Vincenzo feels that the duke betrayed Estella once again and led her to her death, but the others know better; at the very end, they embraced once more.