Nicaea, 325 AD: the Christian Church has been divided by the opposing beliefs of Bishop Alexander and the presbyter Arius, and the schism is becoming violent. Fearing that the divisions within the Church will tear apart his empire, Emperor Constantine convenes a council of bishops to debate the issues and decide once and for all what position will be regarded as orthodoxy and what will be considered heretical. As Constantine’s wife, Fausta, tries to soothe the Emperor’s jangled nerves, he admits to her that he doesn’t care which side wins the debate -- but if the Church doesn’t settle its internal discord on its own, Constantine is willing to spill blood in order to keep his empire in one piece.

The TARDIS materialises just outside the city, bringing the Doctor and his companions to witness the famous Council in person. This is relatively close to Erimem’s own era, and in this time, Alexandria is one of the largest centres of Christian study in the world. Night is falling, but rather than spend it in the TARDIS, the Doctor insists upon finding an inn and living the full 4th-century experience. A few streets away, he and his friends meet a stallholder named Clement, who recognises Erimem as Egyptian and directs her and her friends to an inn run by his friend Gregory -- and advises them to stick close to their own people in case of trouble. Peri and Erimem aren’t remotely surprised to learn that the Doctor has brought them to an era in which trouble is brewing, and the Doctor concedes that, while the ordinary people of this era take their theology very seriously, the bishops are more concerned with their own arguments than with listening to the concerns of the laity. Nevertheless, he insists that Clement was probably exaggerating the extent of the danger -- until he and his friends turn a corner to find themselves trapped between two angry mobs heading straight for each other...

The mobs have been stirred up by a man named Julius, a mercenary in the employ of Bishop Alexander’s clerk, Athanasius. Athanasius is willing to do whatever it takes to ensure that Arius’ heresies are extinguished, and Julius has already killed some of Arius’ followers and stirred up the others into an angry mob. Once they meet Alexander’s supporters, violence will ensue, giving the Emperor little choice but to conclude that Arius’ followers are nothing but troublemakers who will bring down the empire if their heresies are allowed to persist. And if there are a few deaths along the way, then Athanasius is content to believe that it is all the Lord’s will.

The Doctor, Peri and Erimem are caught up in the middle of the fighting, and the Doctor orders Erimem to get Peri to safety while he tries to calm the rioters and end the violence. As Peri and Erimem try to flee out of the mob, someone hits Erimem, knocking her senseless; however, a passer-by helps Peri to carry her friend to shelter, and though Peri doesn’t know whether she can trust this stranger, she has little choice but to accept his help. Shortly afterwards, the legionaries arrive, break up the mob and arrest whomever they can get their hands on. The Doctor claims that he just got in the way and is strictly non-partisan, but the centurion, Caius, places him under arrest anyway.

Erimem recovers to find that she and Peri have been rescued by Arius, the presbyter of Baucalis in Alexandria, and one of the two men whose argument is tearing the Church apart. Peri and Erimem assume that he is here to defend his beliefs at the Council, but he reveals that he is not permitted to attend; he is not a bishop, but a presbyter, and a disgraced one at that. He explains that, after studying the works of Origen, he has concluded that God is eternal, unique and indivisible -- which can only mean that the Christ was brought into existence by an act of Creation and was not himself divine. Bishop Alexander disagrees, claiming that this detracts from Christ’s divinity and thus his ability to offer salvation. Peri can’t help but laugh at the pettiness of the argument, but while Erimem doesn’t understand the theological intricacies of the debate, she has seen -- and felt -- its results for herself. Furthermore, Arius believes that Constantine only wants the argument to be settled and doesn’t care who wins. If Arius is judged a heretic by the Council, he will be exiled or executed, and his supporters will be found guilty of heresy -- and all without Arius’ being allowed to defend himself. Convinced that his beliefs are right, Arius has vowed to continue his teaching even if the bishops judge against him; however, this will bring him into conflict with Constantine, who has waged bloody wars in the past and even killed his own brother-in-law in order to maintain a single, unified empire. Peri is sympathetic, but claims that there is little that she or her friends can do to help; however, Erimem is moved by Arius’ determination to stand up against tyranny and by the kindness he has shown her and Peri, and promises to do everything she can to ensure that Arius gets a fair hearing. By this time, the riot has dispersed, and Peri is confident that the Doctor will be able to talk his way out of trouble; thus, Arius agrees to have someone guide them back to where they met, so that Peri and Erimem can return to the TARDIS and wait for their friend.

Caius reports to Constantine, who is enraged to learn that there has been a violent riot on the very eve of the Council. Caius also reports that his legionaries arrested a man who didn’t seem to belong to either faction, and Constantine orders Caius to bring this man to him for questioning. Once the Doctor has arrived, Constantine dismisses both Caius and Fausta, and speaks to the Doctor alone. The Doctor claims to be an interested but impartial observer to the religious debate, which is Constantine’s own position; however, as he is Emperor, the bishops keep pestering him to choose one side or the other, and he’s heartily sick of it. His only interest is in keeping the public under control, but to do that, he must know what they want. To this end, he needs an agent who can tell him what the people are thinking and how to appease them before he has no choice but to send in troops to pacify them by force. The Doctor agrees to act as Constantine’s agent, and convinces the Emperor that the factions might be more willing to accept him if he’s invited to attend the Council.

Appalled, Peri begs the Doctor to reconsider, and he gives in and asks Erimem to at least concede that the situation in Nicaea is more complex than she realises. Since he has an invitation to attend the Council, he reluctantly agrees to take Peri and Erimem in with him so that Erimem can see the Council in action and get a better idea of what’s happening. Erimem agrees to his terms. The Doctor and his companions thus spend the night at an inn and then attend the Council the next morning; Caius is surprised when the Doctor shows up with two women, but the Doctor quotes scripture proving that Paul supported women in the Church, explaining to the surprised Peri that knowledge of this era’s beliefs is vital if one is to understand the culture. Caius gives in and allows Peri and Erimem to enter the Council along with the Doctor. Constantine then enters to open the Council in person, but while Peri is impressed by his regal bearing, Erimem is angered by the tone of his opening speech; he claims that they are united here, free of the shadow of tyranny, but Erimem believes that Constantine himself is the tyrant, one who brutally removed his rivals to power and now claims that this was God’s will. Nevertheless, she remains relatively silent as Constantine advises the bishops to find a peaceful solution to the strife that threatens to divide their Church.

Once Constantine’s opening address is finished, Athanasius rises to address the bishops, insisting that they must deal with the disruptive elements responsible for this schism before they can repair the damage. Arius has refused to recant his heretical beliefs and has defied his superiors by continuing to spread his teachings and turn people away from the true word of the Lord; thus, Athanasius moves that Arius be cast out of the Church and his teachings anathematised as heresy. Erimem has heard enough, and despite the Doctor’s desperate attempts to stop her, she rises and speaks in Arius’ defence, demanding to know in what way it is God’s will to judge a man in his absence, and all but accusing Athanasius of barring Arius from this Council because he fears that the other bishops will be swayed by his arguments. Athanasius, enraged, reminds his fellow bishops that Arius is not permitted to send anyone to this Council to speak on his behalf, and Constantine orders his guards to remove Erimem. As the legionaries drag Erimem from the chamber, she warns the bishops that they are making a grave mistake in misjudging the mood of the people. The Doctor and Peri, appalled, follow Erimem outside, where the Doctor lashes out at his friend for jumping in without waiting to judge the situation. As far as Erimem is concerned, however, she’s judged the situation perfectly well; the people are being oppressed by tyrants who care only for their own power, and she vows to fight for their voices to be heard.

Angered that the Doctor is trying to stop her from doing the right thing, Erimem pushes away from him and flees into the streets to continue the fight on her own -- and Caius restrains the Doctor and Peri from following, as Constantine has just emerged from the Council chambers in a very foul mood. The furious Emperor interrogates the Doctor and Peri, demanding to know why the man who promised to serve him instead smuggled one of Arius’ supporters into the Council to disrupt its deliberations before they had even begun. The Doctor apologises, insists that he had no idea Erimem would react the way she did, and promises to prevent her from causing any more damage. However, this isn’t good enough for Constantine, as her outburst has already caused confusion and uproar in the Council -- and as long as the Church remains divided, the Empire is under threat. The Doctor promises to find Erimem and talk sense into her, and Constantine grudgingly gives him one day to do so before he sends out his own soldiers. Fausta enters the throne room, curious to know more about Constantine’s visitors, but Constantine dismisses them without letting his wife get a chance to speak to them. The Doctor promises not to betray Constantine’s trust again -- but he has already done so once, and as soon as the Doctor and Peri have gone, Constantine orders Caius to follow them. And if Caius gets a chance to arrest the ringleaders of the riots in the city, he is to take it.

The stallholder Clement is in fact one of Arius’ staunchest supporters, and he is currently visiting Arius to tell him that Peri and Erimem were granted access to the Council. However, he doubts that their voices will be enough, and is proven right when the dispirited Erimem arrives and tells Arius that the bishops refused to listen to her. She has interpreted Constantine’s speech to mean that he has already judged Arius to be a disruptive influence, especially since he had her forcibly removed when she spoke on Arius’ behalf; however, she refuses to give up, and vows to lead Arius’ supporters on a non-violent march to the palace in order to show the Emperor just what his people really believe. Clement, excited by the opportunity, vows to introduce Erimem to the people who can help her, and Arius reluctantly agrees to go along with Erimem’s plan, though fearing that violence may ensue.

Meanwhile, Athanasius summons Julius to deal with Erimem; most of the other bishops are opposed to Arius’ beliefs, but Erimem’s support has complicated matters. Convinced that he is doing God’s work, Athanasius orders Julius to find out all he can about Erimem and how much of a threat she poses.

Peri is unimpressed by Constantine’s attitude, but the Doctor points out that she met him under rather unpleasant circumstances. Peri still sympathizes with Arius, but the Doctor reminds her that he will not be killed, only exiled -- as long as history unfolds on its proper course. If Erimem continues to meddle, however, anything could happen. The entire future of Peri’s civilisation is at stake -- and on a personal level, Erimem could be placing herself in grave personal danger by presenting herself as a threat to Constantine and Athanasius. Unfortunately, the Doctor suspects that she’s already aware of the danger and is convinced that the risk is worth taking. Since Erimem doesn’t appear to be in the mood to speak with him right now, he asks Peri to return to Arius’ lodgings and try to reason with Erimem herself. In the meantime, he returns to the TARDIS just in case Erimem returned there instead -- but on the way, he runs into Fausta, who claims to be on a stroll around the city. The Doctor isn’t fooled, and Fausta concedes that she’s taken an interest in him and his friends. She offers to share some vital news with the Doctor in exchange for his promise to tell her about events in the city, and he reluctantly agrees to do so. Fausta thus gives him a ring he can use to enter the palace at will -- and reveals that her husband sent Caius after him. Caius has followed Peri to Arius’ lodgings, and Constantine will no doubt be sending legionaries to arrest him at this very moment. Appalled, the Doctor rushes off to warn his friends, only to realise that he has no idea where Arius is lodging, and no way to deliver his warning...

Despite Peri’s insistence that she was not helping Caius, Erimem is now convinced that her friend has betrayed her. Unaware that he’s let bigger prey escape, Caius takes Erimem back to the palace, where Peri angrily lashes out at the Emperor for going back on his word to the Doctor and sending out soldiers earlier than he’d promised. She admits that she warned the others of the legionaries’ approach, thus giving them time to escape; she insists that she was only trying to help Erimem, but Constantine nevertheless plans to send both her and Erimem to the dungeons and interrogate them until they confess their true agenda. Before he can do so, however, the Doctor returns, having used Fausta’s ring to get past the guards. Constantine demands to know who Erimem and Peri were meeting, and without hesitating, the Doctor reveals that he’d asked Peri to look for Erimem at Arius’ lodgings. Constantine, enraged to learn that Caius let Arius himself escape, dismisses the suddenly disgraced centurion.

Erimem is appalled that the Doctor gave up Arius so easily, and is furious with Constantine, who seems to be living down to his reputation in her eyes. She refuses to betray Arius, claiming that she is acting for the cause of justice, unlike Constantine; as far as she’s concerned, he is a tyrant who pretends to worship a god of peace and love while resorting to assassination and treachery to maintain his grip on power. Constantine, furious, orders his guards to remove Erimem from his presence, but she struggles free of their grip and flees. The Doctor tries to follow her, but Constantine himself puts a knife to Peri’s throat, ordering the Doctor to stay where he is. Meanwhile, Erimem gets out of the palace, and finds Clement waiting for her outside -- but Clement trips and injures himself. Unable to flee, he pulls a dagger and kills the two pursuing legionaries. Erimem is taken aback by the violence, but Clement insists that he had no choice, and he and Erimem flee before reinforcements arrive.

Constantine accuses the Doctor of betraying him, but the Doctor angrily points out that it was Constantine who broke his word. Constantine nearly kills the Doctor on the spot, but calms down, aware that he needs Erimem’s friends alive in order to predict what she will do next. The Doctor tries to convince Constantine to let him go out and find Erimem, but then Caius arrives and reveals that the two legionaries who were chasing her have been found stabbed to death. The Doctor insists that Erimem cannot be responsible, but while Constantine concedes that only the Doctor can predict where Erimem will go, he no longer trusts the Doctor and thus orders that Peri be held under guard at the palace until the Doctor returns. The Doctor protests, but Peri urges him not to argue. The Doctor thus reluctantly leaves the palace, and once he’s gone, Constantine orders his guards to take Peri to the dungeons.

Clement’s clothes are now stained with blood, and Erimem must buy him replacements so that he can move through the city without being spotted. In the marketplace, she meets Julius, who tells her that people throughout the city are talking about what she did at the Council. Thanks to her, more people are turning to Arius’ side, and Julius asks Erimem if she can introduce him to any of Arius’ supporters so that he can help. Erimem thanks him for his kind words but is unwilling to trust a stranger with such sensitive information, and he thanks her and apologises for putting her in such a position. Erimem continues on to the clothing stalls -- and Julius returns to report what he’s learned to Athanasius, who is disturbed by Erimem’s growing popularity. The people are aware that she is fighting for the cause of justice, and if too many of them begin to support Arius’ beliefs, then Constantine will have no choice but to listen to the mob’s demands. Convinced that he is doing God’s work, Athanasius orders Julius to stop Erimem -- whatever it takes. Julius thus returns to the marketplace and bribes the stallholder to tell him where Erimem went after she bought clothing from him. Fortunately, the Doctor has made his way to the marketplace, and he overhears Julius’ conversation and follows him.

In the palace, Fausta finds her husband in a foul mood. He lashes out at her for allowing the Doctor free access to his palace, but reluctantly concedes that this is little more than he himself had done. Fausta claims to be concerned for Peri’s well-being, but when Constantine scoffs at his wife’s show of concern, she admits that she is curious about the newcomers and wishes to learn more about them. Constantine concedes that Peri’s tongue might be looser if she were treated kindly, and allows Fausta to release Peri from the cells -- on condition that the girl remain in Fausta’s custody at all times. Fausta thus visits the dungeons, apologises to Peri on behalf of her husband, and orders Caius to release Peri into her custody. Peri is surprised but grateful, and she follows Fausta to her rooms while trying to politely evade Fausta’s questions about exactly where she and her friends are from.

Erimem refuses to abandon Arius or the injured Clement, and Julius thus attacks her and strikes her down. Before he can kill Arius, however, the Doctor bursts in and smashes a jar over Julius’ head. Fortunately, Julius only caught Erimem with the flat of his sword, stunning but not cutting her. Once Erimem has recovered, the Doctor turns his attention to Clement, who is more seriously injured; fortunately, the blade did not hit any major internal organs, and the Doctor is able to patch up his wound. Erimem is still unsure of the Doctor’s loyalties, but she listens when he tells her exactly what he agreed to do for the Emperor -- and just how much her outburst at the Council has complicated the situation. She concedes that she was wrong to doubt her friends’ loyalties, but still insists that her cause is just and that she will not abandon Arius to his enemies. She believes that the presence of an assassin proves that she’s getting through to Constantine, but the Doctor doesn’t believe that Julius really serves the Emperor as he'd claimed; however, he does believe that Constantine will push back if provoked, and that bloodshed will ensue. Nevertheless, he is unable to talk either Erimem or Arius out of their intention to stage a peaceful demonstration in front of the palace.

Back at the palace, Fausta plies Peri with wine and questions, but Peri remains vague about the exact circumstances of her travels. Nevertheless, she does assure Fausta that Erimem’s only concern is with seeing justice done, and that she truly believes that Arius is being treated unfairly. Fausta informs Peri that Erimem has misjudged the situation; Arius has not been barred from the Council because of his beliefs, but because of his position. This is a convocation of bishops, and Arius is just a presbyter. Constantine’s presence at the Council is merely a demonstration of his position as Emperor; it is the bishops, not he, who will have the final say in the matter. Erimem is opposing the wrong person, and her actions will only prolong the schism within the Church, leading to further unrest and bloodshed.

Athanasius once again seeks an audience with Constantine, but this time he isn’t carrying a message from Bishop Alexander seeking Constantine’s support at the Council; rather, he’s concerned that one of his servants has failed to return from his “errand.” He does not specify what errand he’d ordered Julius to run, but claims to have heard rumours that Arius’ followers are planning to rise up against the Emperor; Julius’ disappearance may be a sign of worse to come. Worried, Constantine thanks Athanasius for the warning and dismisses him -- but on his way out, Athanasius speaks privately to Caius, advising him to prepare the legionaries to strike should the heretics show signs of revolt. Meanwhile, the Doctor returns to the palace, and, after ensuring that Peri is safe, he tells Constantine that Erimem plans to lead a peaceful demonstration to the palace to show Constantine how many of his people support Arius. Constantine prepares to send out his legionaries to control the crowd, but the Doctor insists that a show of force will provoke bloodshed, and asks Constantine simply to address the crowd in person and show them that he, like them, is a Christian who just wants what’s best for the Church. Peri offers to tell Erimem what Constantine plans and thus ensure that she will not arrive at the palace expecting trouble, and when Fausta supports Peri, convinced that she will not betray them, Constantine reluctantly agrees to put his faith in the Doctor one last time.

Arius’ followers gather at Erimem’s bidding, and despite his injuries, Clement insists upon accompanying them to the palace. Erimem leads the crowd to the palace, chanting Arius’ name as they go. Peri catches up with them before they arrive, and Erimem apologises to Peri for her earlier harsh words. Peri in turn tells Erimem that, while she disagrees with what she’s doing, she will stand by her friend. She also informs Erimem that Constantine knows they’re coming and will meet them in person, and though Erimem remains sceptical, she agrees to see what happens; her intention was never to cause bloodshed in any case, and she and Arius intend to prevent their supporters from getting out of control.

The mob arrives at the palace, and Constantine steps out onto his balcony to address them -- but before he can speak, the palace doors open and Caius leads the legionaries out to confront the mob. Arius urges his people to remain calm despite this betrayal, but the furious Clement tries to attack the legionaries, vowing to die a martyr rather than stand and be slaughtered. But Constantine did keep his word, and, enraged with Caius for taking matters into his own hands, he orders the legionaries to sheathe their swords. Caius reluctantly obeys, and the legionaries stand down. Constantine then addresses the crowd, informing them that he is non-partisan and that he wishes only for the Church to sort out its troubles by itself. He assures the people that he understands and respects the strength of their beliefs, and hopes that the bishops will take it into account -- but whatever happens, it is the bishops of the Church who will decide how the Church is to be run. Peri begins chanting the Emperor’s name, and the crowd joins in, their anger appeased.

Arius is satisfied with this outcome, and Erimem accepts that she’s done all she can to help him. She is still not convinced that Constantine has done all in his power to ensure that Arius gets a fair hearing, but she concedes that the fact that he is not imposing his will on the Council proves that he’s not the tyrant she thought he was. Having seen for herself that he’s willing to negotiate rather than send in his soldiers indiscriminately, Erimem accepts that she misjudged Constantine and that the Doctor was right all along. She begs her friends’ forgiveness, and after an awkward pause, Peri accepts her apology.