The TARDIS materialises in the city of Kiev in the year 1240 AD, interrupting a family dinner at the home of Governor Dmitri's advisor Isaac. Isaac is open-minded enough to accept the Doctor and his companions as friendly travellers, but his frightened servants rush to the governor's palace to tell the governor of the strangers who appeared from thin air. The Doctor and his companions are thus brought before Dmitri, who is desperate for any kind of help; a devilish horde is drawing nearer by the day, and Dmitri fears that his people will be slaughtered. The Doctor knows that he's right, but can do nothing about it. Travel is rare in this century, and the people of Kiev fear that the approaching Mongol hordes are the spawn of Satan, but they're just men. The arrival of Batu Khan's army is a natural part of history and the Doctor cannot interfere. But Dmitri is just as determined and stubborn as the Doctor; he will not stop to using force or torture, but the Doctor and his companions will not be allowed to return to the TARDIS until the situation is resolved.

As weeks pass, Steven tries to help fortify the city for a siege, while Dodo befriends Lesia, the daughter of Dmitri's rather unpleasant advisor Yehven. Yehven has his own plans to save the city, feeling that his betters have betrayed him; Prince Michael has fled and the Church officials seem too frightened to help, preferring to believe that the Mongol hordes will help them deal with the "southern problem". However, Bishop Vasil will not stand in Yehven's way should he find another way to deal with the Mongols. Thus, Yehven and the engineer Taras break into the cathedral late at night, and follow Yehven's map through the catacombs beneath the city, where a dark angel lies waiting to protect Kiev in its darkest hour. But when Taras finds and opens the shining silver casket in which the dark angel sleeps, something monstrous bursts out of it and kills him. Shaken, Yehven flees back to the governor's palace, where he lashes out at anyone who gets in his way. He doesn't care for the Jewish Isaac, and it's just as well he doesn't know that Lesia and Isaac's son Nahum are in love, or that Isaac is secretly translating the holy scriptures into the Russian common tongue, a heretical act in this era.

Steven offers to help fortify the Church of the Virgin for use as a shelter, and finds Taras' body while clearing away rubble. Mykola, captain of the city guard, finds Steven with the body and accuses him of murder; however, the Doctor finds needle-sharp bone fragments embedded in the puncture wounds in Taras' neck, and realises that something unearthly has been unleashed in Kiev. He thus agrees to travel to the Mongols and plead with them to spare Kiev, in order to prevent them from acquiring whatever technology has been loosed in the city. Dmitri agrees to arrange an escort, although the Mongols are said to kill diplomats. In the meantime, Yehven, Isaac and their families must move into the governor's palace for protection, and Steven must remain imprisoned, as things are grave enough and Dmitri doesn't want the people to panic at the thought of something unnatural roaming the city. But that's exactly what's happening, and later that night, Taras' widow Elisabet is apparently visited by her late husband...

Steven is locked in a cell with Olexander, a former monk with a talent for translation. Over a year ago, Yehven asked Olexander to translate a manuscript which had been passed down through his family for generations. It told the tale of a silver casket which descended from the sky in flames; when Yehven's ancestor touched it he was granted a vision of a war in Heaven. He therefore brought the casket to Kiev in the hope that the angel within would protect the city. Yehven became obsessed with raising the angel to save them from the Mongols, and when Olexander objected, Yehven and Vasil conspired to frame him for translating the holy scriptures. Steven is then confronted by Mykola, who has been sent to question him about the TARDIS and torture the information out of him if needs be. Before he can do so, Lesia and Dodo arrive to visit Steven, find that the guards have been dismissed, enter the cell and knock Mykola out. Steven sends Dodo and Lesia back to the palace, while he tries to clear his name; knowing that Vasil and Yehven have worked together in the past, he suspects that Vasil planted Taras' body near the cathedral to frame him. Olexander remains in the cells, refusing to escape before he has cleared his reputation legitimately.

Dmitri learns of Steven's escape, but is wise enough to wonder why the guards were dismissed while Mykola questioned him. He thus decides to appoint Mykola as the leader of the Doctor's escort, to keep him out of the way. The Doctor is initially suspicious of Mykola, but as they journey towards the Mongol hordes, the Doctor eventually comes to accept that Mykola acted out of concern for his city. Mykola, although unwilling to betray his spiritual betters, does tell the Doctor that it was not Yehven who sent him to Steven's cell. After a night in the woods and a close encounter with a pack of wolves, the Russian party continues on, and encounters a lone Mongol riding nearby. Despite the Doctor's misgivings, Mykola urges his men to capture the scout, and rides them right into an ambush. Mykola is spared as he is the leader of the group, while the Doctor is spared due to his distinctiveness; however, the others are killed and left to lie where they fall. The Mongols take the Doctor and Mykola back to their army, and bring them before Batu's cousin Mongke Khan for questioning. Unlike the Russians, the Doctor refers to the Mongols by their proper name, not by the name of the Tartars whom the Mongols have already conquered. Mongke is impressed by the Doctor's wit and respect, and his willingness to argue on behalf of a people who are not his own. He thus allows the Doctor to live -- but he tortures Mykola to death for information on Kiev's defenses.

The dark angel interprets its sensory input in the terms of its own circumstances, and believes it has a mission to complete. It infiltrates the palace in the form of Elisabet, and attacks Nahum in his bedchamber -- but after biting his neck with needle-sharp teeth, it withdraws, leaving him shaken but alive. However, a young Russian soldier is found dead in the corridors, his body in the same condition as Taras'. It appears that the dark angel is killing the people it was supposed to protect. Yehven thus releases Olexander and sends him into the catacombs, to plead with the dark angel on behalf of the people of Kiev. Steven follows them, only to be trapped in the catacombs with Olexander when Yehven locks the door behind him; obviously, he doesn't intend for Olexander to escape and incriminate him. While following Yehven's map to the casket, Steven and Olexander find the real Elisabet's body, and are confronted by the dark angel; however, it will not listen to Olexander's attempts to reason with it. It kills him and turns on Steven -- but withdraws after its initial attack, allowing Steven to live. Shaken, Steven follows the dark angel out of the catacombs to a storage cellar, and sees another door leading out of the cellar into the governor's palace. The creature can move between the palace and the catacombs as it pleases.

Steven turns himself in and tells Dmitri of all that he's seen. While hiding in the cathedral he saw clerics escorting a hooded figure to a hiding place, and overheard a conversation which suggests that Vasil is seeking an alliance with the Mongols to destroy the "threat" of Islam. The guard's murder proves that Steven is not responsible for Taras' death, and Dmitri clears him of all charges and sends his guards to search the church properties in the city for Vasil and the Mongol spy. An outbreak of disease then begins to sweep the city, and Lesia apparently falls into a coma. Yehven, frustrated and guilt-stricken, lashes out at Isaac, blaming him for freeing Steven and bringing these troubles upon them. He thus informs the church officials that Isaac has been translating the holy scriptures, and on behalf of the absent Vasil, one of the clerics sends a soldier to burn the heretical texts. Isaac is trapped behind the flames as the fire spreads, but Steven and a palace guard use the secret passage in the storeroom to get behind the advancing wall of fire and rescue him. The soldier is arrested while trying to set fire to the TARDIS, and claims that he was only obeying his church's orders. Dmitri orders his guards to arrest Vasil, but the bishop has fled the city -- and before they can search further, torches are spotted on the hills. The advance scouts have arrived; the Mongol horde is less than a day's ride away...

As the Doctor rides with the Mongols, he learns that they do not impose their own customs upon the cultures they have conquered; rather, they learn from them. The people of the Chin Empire still live as they always have done, albeit under Mongol rule, and the Mongols have learned from them the secret of wearing silk undershirts in battle so arrows can more easily be drawn out of the skin. But even so, there were fifty million people in the Chin Empire before the Mongols arrived, and only nine million survived the battle. Mongke has allowed the Doctor to live as a curiosity, as he is amused by the Doctor's attempts to use riddles and parables to divert Mongke from Kiev. As the city comes within sight, Mongke finally agrees to at least attempt a bloodless conquest; perhaps the people are frightened enough to surrender peacefully. Mongke thus sends his translator, Abd N-Nun Ayyub, to discuss terms with Dmitri.

Dmitri's guards find the Mongol spy in a building owned by the church, but he is unwilling or unable to communicate with the governor. As the exhausted Dmitri retires for the night, Yehven privately admits to Steven that he's grateful to Dodo for watching over Lesia. In the face of the disaster he has feared for so long, he wants his enemies to understand that everything he did was for the good of the city. But Steven can't forgive him his acts. Meanwhile, Dodo sees Lesia waking at last, but when she turns to leave the chamber somebody knocks her unconscious. The angel then attacks Yehven, and while the guards are diverted searching for it, it slips past them in the guise of Lesia and enters Dmitri's bedchambers. Dmitri survives its attack, but seems badly shaken by the encounter; the next day, his mood veers between the manic and the fatalistic, and when Ayyub arrives Dmitri refuses even to listen to him. Instead, he has both Ayyub and the Mongol spy executed. There will be no peace settlement.

When Dodo awakens, Lesia is missing; it appears that the angel has taken her to the catacombs so it can move about the palace in her form without being detected. Nahum insists upon going to the catacombs to search for her, but when he, Steven and Dodo enter the storeroom, they find another problem. Too many people have died of the disease to bury, especially with the Mongols so close, and the people of the city have been using the burned-out section of the palace as a makeshift morgue. Steven sends Dodo to warn Dmitri that the disease may strike those in the palace, while he and Nahum continue on -- but when they encounter the angel, it transforms into Lesia's form before their eyes, and withdraws into the palace, once again allowing them both to live. Steven and Nahum return to the palace to warn Dmitri that the creature can change its appearance, only to find that Dmitri has apparently gone mad. When Dodo, frustrated, tells the raving Dmitri that he can't just throw his problems away, Dmitri is inspired to fling bodies over the city walls, in the hope that the disease will strike the Mongols.

As Batu Khan himself arrives to oversee the attack on Kiev, Vasil attempts to speak with the Mongols, still hoping to make an alliance against the forces of Islam. But all hope of peace is dashed when the bodies of Ayyub and the spy are flung over the city walls. Vasil, fearing for his life, tells the Khans about the city's dark angel, but they do not fear his stories. Disgusted by his cowardice, they allow him to live and flee, to tell the rest of Europe of the coming apocalypse. The Doctor, realising that he cannot stop the coming slaughter, requests permission to return to Kiev to be with his friends; the Khans agree, recognising in him the honour that Vasil lacked. The Doctor returns to find Dmitri mad, Lesia missing and presumed dead, and the dark angel still on the loose. The creature has shown signs of intelligence in its behaviour, moving from form to form until it could get close enough to attack the governor himself. But what is its purpose? Lost for answers, the Doctor offers to lend his expertise to cure the outbreak of what he believes to be cholera; however, as Dmitri has gone mad, Yehven has become the ruler of the city. Perhaps wishing to be rid of his enemies, perhaps as a last gesture to save them, he has the guards seal them all in the catacombs.

The attack begins. Despite the people's attempts at fortification, the wooden Polish Gate to the west falls swiftly under the Mongols' flames and explosives. The hordes enter the city, killing everyone they encounter. The Church of the Virgin collapses under the weight of its fortified additions, crushing hundreds who have sought sanctuary within or on its roof. Yehven is brought before the Khans, who behead him. Kiev has fallen... and the alien's work is nearly done. Taking the form of a Mongol soldier and then a Russian, it kills the two guards in the storeroom and enters the catacombs, following the Doctor's party.

While discussing the disease with Isaac, the Doctor reveals that in the year 1346, a Mongol army besieging the city of Kaffa will try Dmitri's trick in reverse. To end the siege they will throw diseased bodies over the walls, thus passing the plague on to Genoese merchants, and thence on to Europe, where it will become known as the Black Death and will wipe out one third of the continent's population. Dodo is appalled, fearing that the Mongols learned the trick from Dmitri, and that her casual remark may have set these events in motion. As the Doctor tries to assure her otherwise, they arrive at the casket, and find Lesia still alive. Steven and the Doctor study the casket and confirm that it is an alien travel pod, but before they can investigate further the alien returns and attempts to kill Dmitri. When Nahum steps between them, however, the alien holds back, explaining that potential allies must not be harmed. In other words, the enemy of its enemy is its friend. The Doctor, guessing the truth, orders the others to remain between the alien and Dmitri at all times. He then removes the control unit from the travel pod and returns to the TARDIS with Steven to finish the creature off. However, soon after they've gone, the Mongols arrive in the catacombs. Mongke and Batu allow Isaac to explain himself, and decide to let him and his honourable friends live; however, Dmitri must be executed for his crimes. Dmitri pushes past the others, accepting his fate -- and the alien immediately attacks him...

As the Doctor and Steven emerge from the catacombs, Steven experiences a vision of a war on an alien world. The control unit is a telepathic interface between the alien and the data banks in the travel pod, and it is trying to communicate with Steven, just as it did with Yehven's ancestor when he touched it. The guards on the TARDIS are all dead or fled, and the Doctor and Steven thus enter the ship and wire the control unit into the console to study the travel pod's data banks. The Doctor's suspicions are confirmed; the alien is a lost soldier from a distant and forgotten war between two virtually identical ethnic variations of the same species. Its mission is to infiltrate an enemy bunker, infect the leaders with madness and bring the bunker down from within. This particular casket missed its target, floated through space for aeons, and eventually fell to Earth, damaging its data store and mission profiles. When Taras opened the casket, the soldier assumed that it was in enemy territory, and thus killed him and took a genetic sample from his body to determine the nature of the enemy. This is why it has killed only pureblood Russians, and why it has been working to ensure that Kiev falls to the invaders. Despite an interruption from a pair of Mongols, the Doctor uses the TARDIS to reprogramme the control unit's mission profiles, convincing the soldier that its task is done; it thus stops attacking Dmitri and melts away, and the casket explodes.

The Doctor's work is complete, and he and Steven are reunited with their friends in the ruins of Kiev. Mongke and Batu will allow the courageous Isaac and his family to live, and to care for the broken Dmitri. That family now includes Lesia; Yehven is no longer alive to protest her marriage to Nahum, and the Doctor believes that the creature let her live because the child she is carrying is not pureblood Russian... The Doctor tells the Khans only that the creature was a lost beast struggling to make sense of its surroundings, and then shows them a miracle; he, Steven and Dodo step into the blue box and cause it to vanish into thin air. The people of Kiev have been slaughtered, but the Doctor and his companions have prevented the Mongols from acquiring alien technology, and have helped to save the lives of some honourable men and young lovers. Even amidst the darkness, some hope remains.