The elderly Cressida, a.k.a. Vicki, pays a visit to the Cinder, an entity that resides in the cold, damp crypt of a temple in Carthage. Vicki has told her story to a scribe who thought she was mad but wrote it out for her anyway, and she now wants the Cinder's opinion of her first draft. The Cinder sulks and complains of the cold, its only source of heat a single oil lamp, but Vicki knows only too well what will happen if it gets any more warmth than that...

Vicki's story begins soon after she and the Doctor were joined by young astronaut Steven Taylor, whom Vicki regarded as a big-brother figure. The TARDIS materialises in Regency London, near St Paul's Cathedral, right on the frozen surface of the Thames. It is February of 1814, the river has frozen over in the bitter cold, and the citizens of London are holding the last great frost fair, where the rich and poor mingle among booths selling wares and displaying curiosities. The Doctor and his companions enjoy themselves, although the Doctor is disappointed to find that the celebrated fire-breathing Dagenham Dragon is just a bald fire-eating circus performer. A sleazy Italian gentleman, Hippolitus Valzaki, tries to offer his services as a guide, but the Doctor and Steven see clearly that he's just a sordid gossipmonger and dismiss him. Offended, Valzaki draws a sword from his cane, but a passing gentleman named Sir Joseph Mallard -- the deputy warden of the Royal Mint -- intervenes and sends the sneering ruffian on his way.

Sir Joseph and his young wife, Georganna, befriend the Doctor and his companions and accompany them about the fair. At the end of the so-called City Road they find a collection of curiosities that seems much colder on the inside. The Doctor criticizes the curiosities as flimflam and fakery, apart from the showman Captain McClavity's greatest treasure, a phoenix's egg from the market in Tunis. McClavity claims that the weather turned cold and icy soon after he purchased the egg, and that the cold followed him all the way to London. The captain warms up the crowd with his story and then reveals the egg, which seems to flicker with a cold, blue-green flame. Even the Doctor falls silent at the sight, and Vicki faints dead away, her thoughts numb with cold, convinced that a malevolent eye is staring out of a crack in the egg at her. A friendly woman brings her around with smelling salts, and when she looks at the egg, she sees that it's still intact... unless whatever was peering out at her has sealed up the crack again.

The Doctor thanks the woman with the smelling salts, and is delighted to learn that she's the novelist Miss Jane Austen. Miss Austen is surprised to be identified, since her only two novels have been published anonymously, but the Doctor assures her that her secret is safe with him. Vicki then notices Lady Georganna arguing with her husband; she seems to have become obsessed with the phoenix's egg and demands that her husband purchase it, whatever the cost. Sir Joseph escorts his distraught wife away, and Vicki notices Valzaki following them. At that moment, the fire burning under the roast sheep turns blue-green and loses all its heat, and the Dagenham Dragon staggers towards McClavity's tent, his body wreathed in cold flames. Miss Austen lays the Dragon out cold with a punch to the jaw, and the flames go out. Steven then discovers that, in all the confusion, somebody has stabbed McClavity in the heart with an icicle and stolen the phoenix's egg. And the temperature seems to be dropping even further...

The Cinder seems to be enjoying this part of the story, of perhaps just Vicki's discomfort as she relives it. Displeased, Vicki reminds the creature that she doesn't have to keep visiting it; she could just lounge in the heat of the terrace at home, letting servants bring her rosewater sherbets. Nevertheless, she continues with the story, even as the grumpy and vindictive Cinder tries to rattle her by repeating what the thing in the egg had said to her: "I know where you are."

The frightened fairgoers turn on the Doctor and his friends as outsiders, but Miss Austen escorts them away before someone can set the police on them. When Steven lets slip that he and his friends have nowhere to stay, she invites them to lodge at her brother's house in Covent Garden. Before leaving, Sir Joseph had invited them all to dinner at his home, and Miss Austen supplies her new friends with suitable clothing. She also inquires whether Vicki is visiting London to find a suitable husband and whether Steven is eligible, and the mischievous Vicki plays up Steven's accomplishments at singing, horse riding, fisticuffs and various other diversions of the age. She and her friends then set off for Sir Joseph's home in Devonshire Square, but the temperature continues to plummet in the streets outside...

When they arrive, they find Sir Joseph remonstrating with a chimney sweep, Mr Huggard, who claims to have lost one of his boys down Sir Joseph's chimney. Sir Joseph pays him off and sends him on his way, but Vicki notices that Valzaki is also loitering nearby, watching the house. Inside, Miss Austen spreads word of Steven's accomplishments, and Steven soon finds himself the centre of female attention. While the Doctor and Sir Joseph discuss the odd events at the fair, Vicki notices Lady Georganna shivering by the fire; Georganna's hands are as cold as ice, and she claims to have been chosen by the thing in the egg. After speaking with a servant, Lady Georganna leaves the hall, and since the Doctor and Steven are both busy, Vicki decides to follow her...

The Cinder interrupts, asking if Vicki was scared that the phoenix would come for her. She's shivering now, perhaps from emotion or perhaps from the cold of the crypt -- but she sees through the Cinder's attempts to lure her closer to warm herself by its lamp, and continues her story...

The elderly Vicki remembers being scared but relieved that Georganna had been taken instead of her, and she begins to cry, overwhelmed by her memories, as if it's all happening again in a continuous cycle. The Cinder mocks her, telling her that the cycle always leads to the flames taking what they need. But though it calls her cold and cruel, it's stuck with her version of the story until it can write its own...

The Doctor realises that the frostfire is spreading across London, absorbing heat from everything it touches. Steven is irritated when the Doctor chastises him for throwing out Valzaki before they could learn more, and the men set off to hunt down the rogue -- perhaps at the Cocoa Tree, his favourite gentleman's club. Vicki is put out that she and Miss Austen have been ordered to remain behind, but she then recalls Valzaki mentioning a church, and Jem, the young chimney sweep, offers to guide them to the nearby St Cuthbert's on Wharf. Miss Austen agrees to take a detour on her way home. The streets are cold and deserted, and the flames of Jem's torch are pale and sickly, as if the warmth is being sucked out of them. Poor, homeless people are lying frozen in doorways, and passers-by whisper that the spectre of Winter herself is stalking the streets. Vicki, Miss Austen and Jem then see a sedan chair carried by two frost-rimmed men approach a small bonfire. Lady Georganna steps out of the chair, her face frosted with ice, and the people around the fire drop where they stand, the frostfire sucking the heat and life right out of their bodies. Jem panics and throws his torch at Georganna, distracting her just long enough for himself, Vicki, and Miss Austen to flee.

The Cinder now seems eager to rush the story to its conclusion, as if hoping that this time the ending will be different. Vicki is having second thoughts about spending so much time down in the cold, away from her husband and children, but the Cinder orders her to continue...

Vicki, Jem and Miss Austen run for the shelter of St Cuthbert's, where Jem tries to jimmy open the frozen doors with a crowbar -- but the doors open of their own accord. Miss Austen, exhilarated by the adventure, sends Jem on his way with a sixpence, telling him to fetch the Doctor. Inside, she and Vicki find dozens of people who came to pray for deliverance only to be frozen where they knelt. Valzaki is here, his face lined with frost, and he forces them both up to the bell tower, where they find the phoenix's egg sitting in a nest of frost. Repulsed, Vicki tries to smash the egg; she's still young, with no children of her own, and does not yet sympathise with the Cinder's insistence that the blameless, unborn chick in the egg has a right to be born. In any case, she collapses from the cold before she can reach the egg, but the Doctor, Steven and Sir Joseph arrive with Jem, distracting the egg before it can draw the heat and life out of Vicki. Steven strikes aside Valzaki, who shatters into pieces when he hits the floor, and the horrified Sir Joseph wonders if the same fate will befall his wife.

The Cinder still retains some memories of its old form, and it bitterly reminds Vicki that they're both homeless because of the Doctor. Vicki admits that she never really understood just how different the past was until she chose to stay with Troilus; the Trojans didn't think the same way she did, and she was lucky that Troilus chose to remain with her in Carthage while the other Trojans went off with Aeneas to found Rome. Nevertheless, though she sometimes fears her choice was a mistake, she has accepted responsibility for it -- unlike the Cinder, which still insists that the phoenix chick had a right to be born, though it would then have devoured all life and heat on Earth. The cycle must continue, or so the Cinder claims -- but this is Vicki's story, and she knows better.

Though the egg is vulnerable, the Doctor is reluctant to destroy a unique form of life if it will accept his offer to take it away from Earth. Before he can decide what to do, Lady Georganna sweeps into the room, knocks the others aside with a blast of cold and then sweeps back out, bearing both the egg and her husband. Since she's obviously under the egg's thrall, the egg must need Sir Joseph alive, and when Vicki points out that he works at the Royal Mint, the Doctor realises that the egg wants him to grant it access to the Mint's furnaces -- the hottest fires in London and the ideal hatchery for the phoenix chick. By the time they reach the Mint, the furnaces are burning a cold blue-green and Georganna's two frozen porters are working the bellows. The Doctor storms into the furnace room as if impervious to the cold and gives the egg one last chance to accept his offer of transport to an unpopulated world. Georganna rejects the offer and throws the egg into the furnace -- where it hatches into a spindly phoenix chick that spreads its wings and begins to glow, feeding on the heat of the flames. But it is still vulnerable, and the Doctor and Steven pull the frozen porters away from the bellows while Miss Austen jams the mechanism with Jem's crowbar. As the fires begin to die down, the phoenix catches Vicki's eye as if pleading for mercy, but it showed none to others, so she and Jem slam the furnace door shut. A single spark from the fire catches in Vicki's eye, but the phoenix itself is trapped inside the furnace with no further source of heat, vainly beating its wings against the doors until it and the furnace both fall cold and silent.