When Charley Pollard first met the Doctor, she’d stowed away on the R101 in order to meet a young man named Alex at the Singapore Hilton on New Year’s Eve. For some time, the Doctor has been trying to get her back to 1930 so she can keep that appointment, but for some reason it’s proven extremely difficult. However, he has succeeded at last. Charley meets Alex by the bar, and their evident happiness convinces the Doctor that he was right to change history by saving Charley’s life, whatever it may have done to the web of Time. As Charley and Alex head for the terrace, the Doctor sits in the tea gardens to read through the newspapers, but as midnight approaches he’s interrupted by a rather unfriendly stranger. This is Sebastian Grayle, who has come to gloat over the Doctor’s failure -- for Grayle is immortal, and at some point in his past, and the Doctor’s future, he will kill the Doctor and betray the Earth to alien invaders. Already, they have consumed this world and become the masters of Time and Space; this timeline is an illusion, generated to lure the past Doctor here just so Grayle could look his enemy in the face and gloat.

Grayle departs, satisfied at last, just as Charley returns. When she reveals that Alex’s last name is Grayle, and that he met his grandfather here earlier today, the Doctor realises that Charley’s been used to lure him into this trap. He and Charley retreat to the TARDIS before Grayle’s masters unweave this false timeline, and once there, the Doctor takes readings which confirm Grayle’s claims that the web of Time is unravelling. Charley has been thinking about her survival for some time now, especially following her encounter with Edith Thompson, and has realised that she would have died on the R101 had it not been for the Doctor. However, the Doctor refuses to accept that the laws of Time won’t bend enough to let him save one promising life, and that Time will accept Charley’s survival; Grayle and his masters must be responsible for the storm ripping through Time, and that means that the Doctor must stop them. He thus swabs some saliva out of Charley’s mouth; since she and Alex kissed on the terrace, this gives the Doctor a sample of Alex’s DNA, which the TARDIS traces through the Vortex to the Grayle family home in Britain. In the attic, the Doctor and Charley find an old Roman amphora, which the Doctor decides to take as the starting point. After a bit of research, he and Charley will travel into the past, to save the world.

According to the records in the Abbey of Felsecar, the name Grayle first appears in a British context at a Roman fort in the year 305 AD. Many of the soldiers worship Mithras, a legendary figure who once saved the world from evil by slaying a great demon bull; when compared to this act of heroism, Lucilius sometimes feels that his life lacks purpose. Marcus appreciates the fact that Lucilius truly believes in what he does, but he and Lucilius both agree that Decurion Gralae, who will be the penitent at tonight’s ceremony, is just too intense and strange for comfort. Whoever he prays to privately in his tent, they doubt it’s Mithras...

The TARDIS materialises in the fort, and the Doctor and Charley emerge in search of Sebastian Grayle -- or rather, Decurion Sebastius Gralae. Gralae is praying in his tent, asking to be shown the truth, when they find him and interrupt him to warn that the Picts are preparing to revolt. Despite the urgency of the warning, however, Gralae insists that he must officiate at the temple. Before leaving, he urges Charley to leave the fort at once, for this is no place for a lady such as herself. The Doctor follows Gralae to the temple, leaving Charley to search the tent for further evidence -- and Charley soon finds an alien communicator which enables Gralae’s alien masters to speak to him.

At the temple, Marcus and Lucilius welcome the Doctor to their ceremony while Marcus fetches the ritual sword, a rustless weapon with which it’s claimed Mithras himself slew the demon bull. However, it’s clear that this ceremony isn’t going to go smoothly, for Gralae is moving the statues in the temple into different positions, for some reason. Marcus tells the Doctor that Gralae, a native of Londinium, has some very odd ideas -- according to him, it’s the demon bull and not Mithras who was the powerful one.

As Lucilius opens the ceremony with announcements about marriages and such, Charley calls the Doctor out of the temple to show him the communicator. The device is capable of transmitting both sound and picture, but Gralae has set it for sound only, suggesting that he still has doubts about his masters. Also, he’s still wearing his armour, which implies that he is not yet immortal and invulnerable as he’d claimed. There may still be a chance to turn him from the path of evil. But before the Doctor can come up with a plan, the communicator activates, and an alien voice demands to know if the sacrifices are ready; the lives in the temple will be forfeit in exchange for the first installment of Grayle’s power. The Doctor will have to act quickly if he is to prevent Grayle from becoming the evil immortal from Singapore...

In the temple, Lucilius hands over the ceremony to Gralae, who dashes the amphorae of bull’s blood to the floor, dashes out of the temple and seals everyone else inside. The Doctor and Charley arrive just in time to see a blazing light envelop Gralae; pure psionic energy, phased through the event horizon of a black hole, is being channeled into his body. The Doctor and Charley unbolt the doors of the temple and warn Marcus and Lucilius of the danger, and when the soldiers see the sky blazing the colour of blood they accept that they must retreat from this unnatural threat. Marcus and Lucilius thus gather their men together and evacuate the fort -- but as they reach the gates, a strange metal demon appears before them, firing bolts of light and threatening to exterminate them all. The Romans overpower and destroy the creature, and continue their evacuation.

The Doctor and Charley escape moments before the fort is completely destroyed. The Doctor then uses the TARDIS to trace the transmat beam’s point of origin, which is in the Ordinand system; the alignment of the planets in that system, and the nearby black hole through which the beam passed, means that Grayle’s masters will be unable to contact him again for 750 years. As the Doctor sets off to prevent this, Charley realises that Grayle’s hatred of the Doctor must stem from the Doctor’s interference in his affairs -- but the Doctor would never have interfered had Grayle not contacted him to gloat. The Doctor advises her not to try reasoning out temporal paradoxes such as these; such attempts can drive one mad, or worse. There’s an old Gallifreyan nursery rhyme about a mythological bogeyman called Zagreus... but that’s not important right now, as they have Grayle to deal with. Travelling 750 years into the future will take them to the court of Edward the Confessor. Edward and his wife Edith kept England safe for decades by promising the throne to everyone who wanted it; thus, instead of attacking a kingdom they each assumed they would inherit, their enemies wasted their time fighting each other. The TARDIS picks up a transmission from the Ordinand system to the royal court, and the Doctor and Charley prepare to meet the royal family -- taking with them a radiation detector, which is picking up a power source somewhere in the vicinity.

Edward and Edith have just banished the former Earl of East Anglia to the wilds of Scotland as punishment for abusing his former subjects. Now they need a new earl, and there are plenty of candidates, and the foremost is Bishop Leofric of Exeter, who arrives at court with gifts of jewellery, forged from a comfortably warm metal which miners in Exeter have found in new shafts opened to Leofric’s instructions. Edward and Edith thus invite Leofric to dine with them at court. Dinner passes well, although the king and queen seem to be coming down with chills, perhaps because the court has been built so near the river. The Doctor and Charley also arrive at court, and Edward warmly welcomes his old friend, the Reverend Doctor of Bruge; however, Edith is somewhat less pleased to see them. The Doctor introduces Lady Charlotte to the king and queen, and leaves her to talk with them while he speaks with Bishop Leofric... who once went by the name of Sebastius Gralae.

Grayle has changed since he last met the Doctor, and not for the better. He spent decades in penitence, only to watch his fellow monks age and die; he has married twelve times, only to watch each wife wither away. The world goes on, and he’s been trapped here, unchanging, for centuries, hating the man who brought him to this. Soon, however, his masters will strip away this world; he now believes them to be angels of the Lord who will punish his enemies. The Doctor, knowing that Grayle likes to gloat -- and knowing that he can’t risk harming the Doctor while the king and queen are watching -- tries to learn more about his masters’ plans, but then Charley collapses, burning with fever, and the Doctor realises that she’s been poisoned.

The Doctor hurries Charley back to the TARDIS, where the nanites in the console room restore her to health -- but in his haste, he leaves the door open, and Grayle follows him in. Grayle no longer has any compunction about harming a lady, for he’s seen too many men and women go to dust to care about fleeting mortal lives. He prepares to kill the Doctor, but the Doctor keeps him talking until Charley recovers fully and hits Grayle with the TARDIS hatstand, knocking him out. She suggests transporting Grayle to a deserted planet to get him out of the way, but the Doctor refuses; he needs to know who Grayle’s masters are, and right now Grayle is the only lead the Doctor has. In any case, he pities Grayle, who has lived for so long without changing that the passage of Time has killed everything good within him. He and Charley must get Grayle out of the way and find out what’s so important about the “special metal” which his masters have ordered him to mine.

Fearing the damage Grayle could do inside the TARDIS, the Doctor and Charley carry him out, only to be caught by Edith and her guards. The Doctor tries to explain himself, but Edith has him and Charley arrested and sent to the dungeons. The Doctor knows that she doesn’t really believe he harmed “Leofric” -- she’s just upset because when they last met, due to a series of misunderstandings, the Doctor ended up ditching her at the altar. He hopes to get word to the King -- but Grayle then arrives and informs them that Edward has fallen ill, and that he, “Leofric”, has been given leave to take the Doctor’s confession. Grayle searches the Doctor to ensure that he has no tricks up his sleeve, and finds the radiation detector, which buzzes when he picks it up. Setting this aside, Grayle begins to torture the Doctor -- and although the Doctor can stand the pain, he loses his temper when Grayle prepares to torture Charley as well.

Edith arrives, but doesn’t free the Doctor; rather, she compliments “Leofric” on his ability to get past all of the King’s safeguards and poison him. However he’s managed it, soon Edward will be dead and Edith will reward Grayle for his ingenuity by bearing him a child who will inherit the throne. The Doctor offers Edith the radiation detector as a gift of his own, but she isn’t impressed with the dull, silent object at all. However, moments later, she faints dead away, and Grayle exultantly reveals that the jewellery which he gave to Edith and Edward was made of the strange new ore, and that his masters have instructed him in its use. In its pure form it can kill slowly over months, but when refined, the ore becomes a warm new metal which can kill within hours, and only Grayle can handle it safely. The Doctor realises that Grayle’s “gifts” were forged from plutonium -- and soon, 100 barges will arrive at court, loaded with enough plutonium to power the machine which Grayle has constructed to bring his masters to him.

Charley is horrified to see the Doctor fling Grayle over the edge -- but Grayle is immortal and invulnerable, and thus survives the plunge into the river. The Doctor knows that he’ll flee from the country and lie low -- for decades, if need be -- until the next opportunity to summon his masters arises. However, the fact that they needed a machine to summon them here suggests that they no longer have the power to transport themselves. The Doctor and Charley destroy this machine, and supply the court with anti-radiation pills and instructions on how to dispose of the plutonium, and then depart; however, back in the TARDIS the Doctor collapses, finally feeling the effects of the torture he suffered at Grayle’s hands. He’s uncharacteristically quiet as Charley tends to his wounds, and she eventually forces him to admit that he didn’t have Grayle’s immortality in mind when he threw him from the roof; for a moment, he could only remember Charley’s pain, and he just wanted Grayle dead. What really upsets the Doctor is the pettiness of Grayle’s ambitions; he has become evil, and is ready to tear apart all time and space, all because of the terms of his father’s will.

The TARDIS leaps ahead to Buckinghamshire in the year 1806. As is the fashion, the Doctor must wear a rapier, and he chooses the rustless sword which Charley took from the temple of Mithras. He and Charley emerge to find themselves in a cave system, and a few minutes’ exploration takes them to a golden door which opens into a dungeon; however, most dungeons don’t have dining tables stocked with fine claret. They’re in Wickham Caves, one of the homes of the notorious Hellfire Club which caused them such trouble in 21st-century America. This is its original form, the gentlemen’s club attended by such historical figures as Francis Dashwood, the Earl of Sandwich, and, as the Doctor recalls, American President Benjamin Franklin... The Doctor also notes a collection of artefacts from the Minoan civilisation, indicating that Grayle hasn’t lost his interest in the legend of the demon bull. He and Charley are then confronted by Sir Richard Martin, a guest of the caves’ owner, and cover their trespass by claiming that they were forced to flee when the local guides leading them on a tour of the caves attempted to rob them. As they claim to have just arrived in the area, Martin offers to lead them back to his host -- Sir Sebastian Grayle.

Martin’s daughter Lucy watches, bored, as Grayle sets his falcon to hunt. Though courting her, he shows little interest in social graces, and believes that this world exists only to service him. He is a member of the Hellfire Club -- and tonight’s meeting, to celebrate the turn of their calendar, will be an occasion to remember. Martin then arrives with the horned goblet which Grayle needs for the ceremony, and seems shocked to find Grayle alone with his daughter -- but Grayle cares nothing for polite appearances. Oddly, he seems delighted when Martin introduces his new companions, and politely invites the Doctor and Charley to dine with him. The Doctor accepts, believing that Grayle is feeling overconfident; perhaps he will once again gloat too much and reveal his plans. However, the Doctor has underestimated his enemy. During dinner, Grayle takes issue with one of the Doctor’s comments, twists his innocent words and actions into insults, and finally challenges his enemy to a duel. He doesn’t just want the Doctor dead -- he wants to drag the Doctor down to his level, by killing him using the manners of history.

Given the choice of weapons, the Doctor chooses swords. The opponents prepare for their duel in the hundred-acre field, as Charley watches, upset; how can the Doctor hope to win a contest against an invulnerable opponent? Martin refuses to watch, claiming to be horrified by this violence, but Lucy attends, having deduced that Charley and the Doctor did not come here by accident. As Charley is clearly more than she seems, Lucy admits to her that she does not intend to accept Grayle’s proposal -- but that she has other reasons for remaining at his home. Martin arrives and tries to convince the ladies to leave, but Charley, although upset, refuses to abandon the Doctor, her best friend.

The duel begins. The opponents stand back to back, take ten paces, turn and engage. Grayle is now even older than the Doctor, and he’s had time to practice -- but he’s overconfident, as he assumes he can’t be harmed. He assumes incorrectly -- for the Doctor nicks him slightly with his blade, and the touch of the weapon burns. It’s a rustless sword, the one with which Mithras killed the original demon bull, and its touch draws the alien energies out of Grayle’s body, leaving him vulnerable and mortal once again. If the Doctor holds it to his throat long enough, he’ll feel every one of his 1400 years of life. The Doctor gives him another chance to repent; he knows how Grayle felt as a decurion, dissatisfied with his empty life and wanting the freedom to seek something more, but he won’t let Grayle take that freedom at the expense of the world. Grayle refuses the Doctor’s offer of mercy and draws a hidden dagger, trying to stab him. The others intervene, but Grayle takes Lucy hostage and rides away to safety -- taking with him Lucy, whom he has always intended to sacrifice to his masters.

The Doctor questions Martin, who is desperate to save Lucy and deduces where Grayle will have taken her. Earlier, Grayle had shown Martin a device which he claimed would change the country forever -- in the Hellfire Club’s dungeon in the caves. Grayle has blocked the cave entrance with a rock, but Martin knows of another way in, through the chimney leading to the fireplace in the meeting hall. As they approach the chimney, the Doctor explains to Charley that the rustless sword is made of pure iron, which is often used as a defense against magic; now he thinks he knows why.

The journey through the time corridor has weakened the Nimon, who require their binding energy oscillator to drain the life out of their victims. However, when Grayle tries to fetch the oscillator -- the horned goblet which Martin fetched earlier -- he finds that it’s gone, along with all of the other treasures he’s collected throughout the centuries. “Richard Martin” is not the real Richard Martin at all; he and his “daughter” Lucy are con artists who intended to rob Grayle and leave him at the altar. Enraged, Grayle sets off after Martin, leaving Lucy, Charley and the Doctor locked up with the Nimon.

Knowing that the Nimon need to keep their victims alive to feed, and are too weak to harm them seriously, the Doctor takes the opportunity to examine their travel pod, and discovers that although it draws energy from Grayle’s machine, it can be piloted from within. He explains to Charley and Lucy that the Nimon travel from planet to planet, consuming them as they go -- but Earth is a vital nexus in time and space, rich with life and radioactive ores, and from here they can spread out in several directions at once, thus conquering all of time and space. A scout must have arrived on Earth centuries ago, offering trinkets and secrets of power to the gullible, only to be killed by the man who became known as Mithras; though he wouldn’t have understood that he could earth temporal energy through a sword of pure iron which had passed through a black hole, he knew a magic weapon could kill a magic beast. Despite this setback, however, the Nimon didn’t give up trying to get to Earth, and eventually they found a man who would serve them. Now, Grayle has finally opened the way for them, and although these few scouts are currently weak enough for the Doctor to taunt and ridicule, soon they will regain their strength and their fellows will swarm through the gateway in numbers great enough to overwhelm the Earth...

Grayle locates Martin, seizes the horned goblet, and flings the terrified con man down the chimney. The fall into the dungeon kills him, but the Nimon are distracted by the body falling out of the fireplace, and the Doctor takes the opportunity to pocket some of the gold coins which Martin had stolen. He then ushers Charley and Lucy into the travel pod and pilots it out of the dungeons, into the caves next to the TARDIS. As long as Grayle’s device operates, more Nimon can travel to Earth from the Ordinand system -- unless something blocks the tunnel. The Doctor thus pilots his TARDIS into the time corridor, despite the risk -- for in Singapore, the successful Grayle claimed that the Doctor died at a distance without Grayle getting the chance to gloat over his body, and this may be why...

Grayle arrives with the goblet, too late for the Nimon to consume the Doctor and his friends -- but the other members of the Hellfire Club have arrived, right on time. The Nimon feast on the terrified men and women, and prepare to summon their fellows from the Ordinand system, only to find that the corridor has been blocked. Furious, Grayle and the Nimon locate the travel pod and pilot it into the time corridor, materialising aboard the TARDIS. The Nimon are now strong enough to pose a serious threat, and the Doctor thus gives Lucy a message for Charley and tells her to operate the door controls. As Grayle and the Nimon are concentrating on him, they fail to see what Lucy’s doing, and when the doors open the Nimon are swept out into the Time Vortex -- as is the Doctor. Grayle, however, manages to close the doors -- and when he works out how to operate the TARDIS communicator, he can receive instructions from the Nimon and pilot the TARDIS out of the corridor. It appears that he’s won, but he feels cheated by the fact that the Doctor will never know it -- thus, once the Nimon control Earth, and have restored Grayle’s immortality, he intends to ask them to construct a false timeline just so he can lure the past version of the Doctor to him and gloat over his victory. Charley mourns the Doctor’s death, and Lucy gives her time to grieve before telling her the Doctor’s last words -- “fast return switch, three times fast.” Realising that the Doctor had a plan after all, Charley asks Lucy to distract Grayle while she carries out the Doctor’s instructions. Grayle eventually realises that he’s being tricked, but by that time the TARDIS is in motion. Charley claims not to know where they’re going, but Grayle pulls out his flintlocks anyway; wherever they end up, Charley and Lucy will be his hostages.

Normally the Doctor would never be able to survive an unprotected fall through the Vortex, but he’s prepared for this; before the pod materialised on board his TARDIS, he programmed the console to generate a tunnel to a certain set of co-ordinates. Using Gallifreyan meditation techniques to focus on his destination, he arrives at his destination before the Nimon, falling out of the sky before the amazed Marcus and Lucilius, in the Roman fort where this all began. He warns Lucilius that the legendary demon bull is returning; Lucilius must take the rustless sword from the temple and hold it back. At last, Lucilius has a chance to battle true evil. As Marcus prepares for the battle, the Doctor heads for Decurion Gralae’s tent, having finally realised the truth; for Gralae is praying not to the demon bull, but to Mithras, pleading to be shown a sign before committing himself to serve the darkness. The Doctor calls him out of his tent to see that his fellow soldiers are being called to arms, to fight the demons which he’s summoned. Believing that the Doctor is a messenger of Mithras, Gralae begs forgiveness for his actions -- and the Doctor thus gives him the gold coins which he’d taken from Martin’s body and which Martin had stolen from the future Grayle. At last, Gralae can buy out his commission, purchase his own villa... and marry his beloved Julia, whose father would not allow her to wed a man with such poor prospects. Here at the start of it all, Grayle’s evil was born from love.

The Nimon arrive at the fort, and Lucilius leads the attack, determined to rid the world of this evil as Mithras did before him. Armed with the rustless sword, he cuts down the Nimon, who remain defiant and battle to the last, vowing to consume the world and kill all humanity. In the end, it is the Nimon who die; five beasts lie dead in the temple, and five dozen Romans with them. But despite the carnage, Lucilius knows that good has been done here.

The TARDIS materialises in the fort, and Grayle emerges to find the Doctor standing next to his younger self. Decurion Gralae is horrified to see his future self threatening the lives of women, and is appalled when Grayle casually dismisses his love for Julia as foolish; she died in a season, and Grayle has forgotten what she even looked like. Gralae was willing to give up everything to spend his life with her; Grayle did, and found it wasn’t worth it. Now he can have power over everything in existence, and he will start by killing the Doctor while he is still mortal. Gralae realises that his future self does intend to destroy all the peoples of the Earth -- and before Grayle can shoot the Doctor, Gralae stabs him to death. He will not allow himself to become this person, an inhuman being who had lost all knowledge of honour and love. The Doctor orders Gralae to destroy the communication device which his masters had given him, and to evacuate the fort before the transmat beam hits.

History is back on course, which means that the Doctor and Charley’s encounters with the future Grayle never happened. The Doctor assures Charley that Time will sort it all out; her head is filling with new memories, but she’ll remember some as dreams and some as stories. The Doctor takes Lucy back to her own time to arrange her partner’s funeral, and then takes Charley back to Singapore, 1931, to see in the New Year properly. Again, the TARDIS seems reluctant to go there, but the Doctor dismisses this as unimportant; he’s sure that the temporal disruption has been resolved, and that his rescue of Charley had nothing to do with it. Sadly, he’s wrong. Later, when called upon to explain his actions, he will insist that he truly believed the Nimon to be responsible for the storms ripping apart Time -- but in fact, they merely took advantage of pre-existing conditions created by the Doctor himself. And as he and Charley see in the New Year in Singapore, they’re unaware that there is far worse to come...