The Doctor receives an anonymous tip from Gallifrey warning him that a problem he’s had his eye on for some time is about to resurface in Prohibition Chicago. Just as he’s about to set off, however, he receives a telepathic call for help from his former companion Romana. He thus hurriedly drops Benny off on a primitive planet in E-Space without briefing her properly, giving her a homing beacon which she can use to summon him in case of emergency. She soon discovers that she’s arrived during a tense time in this planet’s history. Some time ago, the peasants of this world overthrew their evil Lords with the help of a stranger from another world, and ever since, they have been trying to wrestle greater independence from their rulers. Benny visits the remains of the Lords’ Tower to investigate further, and finds it to be the wreck of an ancient spaceship. Inside, she finds the body of a girl with her throat cut, and is attacked by a vampire. She drives off the vampire with an herbal wreath and flees. Outside, on a plain littered with the bones of a giant bat-like monster, she is confronted by an aristocratic blonde woman who warns her not to meddle with the past. Benny returns to the nearby village and tells her story to the headman, Ivo -- who is horrified to learn that the vampire Lords have risen again.

In Chicago, the Doctor opens a speakeasy called “Doc’s Place”, intending to establish a neutral area where the warring gangs of Chicago can meet on equal terms. He refuses to tell Ace what he’s really after, however, telling her that if she thinks too hard about what they’re looking for, it will come and find her first. The Doctor bribes Mayor Bill Thompson and police captain Dennis Reilly to leave his joint alone, and soon attracts the attention of Al Capone himself, who hires honest private eye Tom Dekker to check out the newcomer. Dekker shows up at Doc’s Place just in time to help the Doctor and Ace drive off Pete Gusenburg’s enforcers, and when the Doctor learns that Dekker is here on behalf of Capone, he asks Dekker to set up a meeting between them. Gusenburg sends his goons to the meeting to kill them both, but they survive, and the Doctor convinces Capone not to strike back; another party is trying to stir up trouble, and won’t be satisfied until he’s turned the city into a bloodbath. Capone agrees to hold a peace conference with his rivals, but as the gang leaders talk, dividing up Chicago between themselves, a tall thin man wanders through the lobby, spreading insinuations and mistrust amongst the gangsters’ hired guns. Even Ace walks away from the conference convinced that Capone can’t be trusted, although she’s unsure why…

Benny leads a party of villagers to the Tower to prove her story, only to find that young Gerda’s body has vanished. As the skeptical peasants return home, they find the body of a Black Guard -- one of the Lords’ enforcers -- lying in their path, drained of blood. A patrol of Black Guards finds them grouped around the body, but before Captain Varis can arrest or kill them all on the spot, Benny convinces him -- at gunpoint -- to back down. She and Ivo return to the village, where they find Gerda’s mother Magda waiting at the inn -- with Gerda. Magda believes that her daughter has been wandering lost in the woods, but when Benny confronts Gerda with the herbal wreath, Gerda transforms into a vampire and kills her mother. Ivo stakes her, but decides to hide the body, for fear of disrupting the coming peace talks between the peasants and the Lords. The next day, Benny returns to the plains to study the remains of the alien monster, and finds that some of its bones have been removed. She is then captured by rebels and brought to their leader Kalmar, who questions her and is delighted to learn that the Doctor sent her. The blonde woman then shows up and introduces herself to Benny as Romana. She has been trying to organise peace talks between the peasants and the Lords, but it seems that a third party is acting to keep the classes divided. And now it seems that the taint of vampirism has not been driven from the planet after all…

The truce in Chicago nearly breaks down when the thin man openly shoots a number of public figures, mostly Capone’s enemies. Realising that he will be blamed, Capone intends to strike first against his enemies, but the Doctor manages to stop him from doing so, convincing him that someone is trying to get the gangsters fighting amongst themselves so he can move in and take control once they have wiped each other out. While Ace and Dekker investigate the other two murders, the Doctor questions the only witness to Tribune reporter Jake Lingle’s murder -- a beat cop named Anthony Ruthy, who has been having terrible nightmares ever since he saw the killer vanish into thin air. Meanwhile, Ace gets on the wrong side of thug Tony Ricotti while investigating the murder of Capone’s rival Hymie Weiss. With the help of the thin man, Ricotti lures her to one of Gusenburg’s brothels, slips her a Mickey Finn and tries to rape her. The Doctor and Dekker track her down just in time, and Dekker shoots Ricotti.

The peace talks between the Lords and peasants reach a stalemate, until Benny suggests that they form a two-tier parliamentary system, with a house of commoners and a house of Lords. The progressively-minded Lord Veran agrees to discuss the suggestion further with the young rebel Tarak, but just as it seems that progress is being made, Lord Yarven -- a traditional Lord in more ways than one -- is contacted by a thin man who urges him to take action against Veran. Yarven kills Veran with a scythe, a peasant’s weapon, and Romana, Benny and Tarak are forced to flee from the vengeful Lords -- only to find that Kalmar has been killed with a Lord’s sword. Benny and Romana realise that a third party is trying to set the Lords and peasants against each other, but Tarak insists that the Lords killed Veran to stop him from conceding to the peasants’ demands, and calls on his fellow rebels to rise up. Realising that a bloodbath is inevitable, Romana and Benny attempt to contact Sargon, the sole remaining Lord who is sympathetic to their cause. On the way to Sargon’s castle, they are attacked by the vampires Zargo, Camilla and Aukon, whom Romana once saw crumble to dust. Sargon intervenes, helping them to escape, but they have no way to warn the others that they are walking into a trap -- and as the Lords and peasants do battle in the forest, they are all set upon by vampires led by Yarven.

The Doctor finally explains to Ace that he’s hunting the evil Agonal, who takes delight in chaos and has intervened throughout history to make bad situations worse. At Agonal’s urging, Mayor Thompson orders the police to crack down on the city’s gangsters, and Gusenburg makes yet another attempt on Capone’s life. Capone survives, and this time, Agonal convinces him to take his revenge with what comes to be known as the St Valentine’s Day Massacre. Agonal then convinces Capone that the Doctor bribed his bodyguards to kill him, and Capone invites the Doctor to a party to discuss the situation, only to beat his own bodyguards to death with a baseball bat before the Doctor’s eyes. The Doctor realises that Agonal is watching, and before Capone can turn on him, he challenges the evil immortal to show his face. When Agonal does so, the Doctor pulls out a force field sphere imbued with sensations of Agonal’s past feasts -- a lure which Agonal finds irresistible. But just as the Doctor is about to entrap him, Ace and Dekker arrive to rescue him, breaking his concentration and allowing Agonal to escape from Chicago. Agonal’s abrupt departure breaks his hold over Capone, and the Doctor moodily returns to Doc’s Place with Ace and Dekker. But Reilly and his policemen storm the speakeasy with orders to kill them all, and they must retreat to safety in the TARDIS, Dekker included.

Romana finds Sargon to be a charming man with an extensive knowledge of vampirism and its effect on his planet’s history. Benny, however, finds him a bit too good to be true, and convinces the reluctant Romana to help her investigate further. Behind a painting of the Great Vampire in Sargon’s museum, they find a hidden door leading to a secret laboratory, where Sargon has reconstituted the remains of Zargo, Aukon and Camilla -- and is trying to do the same to the Great Vampire, using the bones he removed from its burial site. When Sargon confronts them, Benny sends a distress call to the Doctor via her SPATAB, only to realise that Sargon allowed her to do so in order to lure the Doctor into a trap. Sargon is Agonal in another guise, and, having stirred up further bloodshed on this planet, he now intends to revive the Great Vampire and release it upon the Universe. To restore it fully to life, he will feed it the blood of Time Lords.

The Doctor arrives to find the peasants and Lords recuperating from the vampire attack. Yarven is nowhere to be found, and Tarak and Varis are dead, but under Ivo’s leadership the surviving Lords and peasants join forces against their common enemy. The Doctor leads them to Sargon’s castle, where he once again tries to capture Agonal -- but to his fury, Agonal is swept away at the last moment by a Timescoop operated from Gallifrey. Dekker shoots the attacking Zargo, Camilla and Aukon through the hearts with his .45 automatic, but before going down, Camilla stabs him with a poisoned dagger. The Great Vampire begins to wake, but the Doctor sets Agonal’s laboratory equipment to overload, blowing up the castle, exposing the Great Vampire to sunlight and destroying it for a second time. Ace puts Dekker in cryogenic stasis in the TARDIS, and as Benny loads her archaeological equipment back on board, the Doctor advises the surviving Lords and peasants to learn the proper lesson from their experiences, and work together in future.

The Doctor then takes his companions to Gallifrey to seek medical help for Dekker and find out what happened to Agonal, but upon emerging from the TARDIS they are arrested by Captain Zorell of the Chancellery Guard. Romana, Benny and Ace are locked up while the Doctor is taken for questioning by the Committee of Three, an alleged security council made up of Time Lords whose careers have been ruined by the Doctor’s meddling. Their leader, Rath, is Chancellor Goth’s younger brother and was one of President Borusa’s staunchest supporters; the Doctor is responsible for destroying both of his heroes. The Three attempt to torture the Doctor under the mind-probe, but Benny, Ace and Romana break out of captivity, force their way into President Flavia’s office and warn her of the danger. Castellan Spandrell rescues the Doctor and arrests Zorell, but the Three flee before he can arrest them as well.

The Doctor realises that the Three sent him the anonymous tip so he would track down Agonal for them; they intend to set Agonal’s power against the mind of Rassilon, and free Borusa from his eternal imprisonment. The Doctor takes Romana to the Dark Tower in the Death Zone to stop them, but the Three have already released Agonal, despite the danger that Agonal may become invincible if he consumes Rassilon’s mind. As the two immortals fight, the Three use a Time Gun to free Borusa from his imprisonment -- but once free, Borusa reveals that he has seen the error of his ways, and refuses to help the Three in their quest for power. Instead, he joins mental forces with the Doctor, Romana and Rassilon, and helps them to crush Agonal out of existence. The demoralised Three return to the Capitol to face judgement, while Rassilon sends the repentant Borusa to another plane of existence.

Romana decides to remain on Gallifrey, while the Doctor returns to Chicago with Ace, Benny and Dekker. Time Lord technology has ensured that Dekker will live a long and healthy life, and since Agonal’s departure has caused things to cool down in Chicago, the Doctor leaves Doc’s Place in Dekker’s capable hands. As he and his friends celebrate their victory, Benny opens the crates containing her archaeological equipment -- and Lord Yarven, who had been sheltering inside, leaps out and flees into the streets of Chicago. The Doctor, however, assures Benny that there’s no need to worry; he took care of that problem long ago. Meanwhile, back on Gallifrey, Romana is contacted by a Time Lady named Ruath, who has made a study of vampirism and wants to speak to the Doctor about something rather important…