Young Steven Keyser unsuccessfully attempts to assassinate Sherilyn Harper, the head of the right-wing New Brittania party, and his associate Walter Jacobs flees as security patrols close in. But the monster that Steven had sensed in Harper gets to him before the security guards do. Some time later, talk-radio jockey Mick Thompson is startled when a strange man called the Doctor walks into his studio booth in the middle of a broadcast and speaks directly to the listening Walter, telling him that he's right; there is a monster out there. Walter calls in and tells him that he's going to do something about it, but refuses to explain and hangs up without telling the Doctor where he's calling from. What Walter doesn't know is that with the help of Paul Tanner, a hacker friend from her old neighbourhood, Ace has traced his call and located the hotel room where he is staying. The Doctor, meanwhile, continues trying to explain to Walter that he hasn't told the authorities of his plans and that he just wants to prevent Walter from doing something he can't take back; but Thompson has recovered from the shock of having his show interrupted and openly mocks the Doctor's claim that there's something evil out there that must be fought. The Doctor queries Thompson's attempts to belittle that which frightens him, only to realize that Thompson has played a commercial for his new book over the Doctor's attempt to defend himself. Ace then calls in, reports success to the Doctor and has a bash at Thompson's bullying while she's at it. As the Doctor leaves, Thompson claims to be stunned at their nerve; imagine going on the air to fling about insults rather than trying to talk sense!

Ace finds Walter hiding in his hotel room, but he pulls a gun on her and forces her to talk until he's sure that she's clear of the monster that Keyser taught him to hear. She tries to convince him that she's here to help, but when she tells him who sent her and he realizes that the man on the radio was the legendary Doctor, he flees in terror. The Doctor finds him in the lift, but Walter is too frightened to listen to him; Keyser had shown Walter classified files about the Doctor, and Walter flees before the Doctor can say anything to him. Having lost his one lead, the Doctor returns to the psychiatric ward where Keyser is being held to question him further. Keyser's encounter with the Fearmonger has left him a nervous wreck; he will be consumed with terror for the rest of his life, and even the Doctor can't help him. All he can do is try to find and stop the Fearmonger before it destroys any other lives, but it won't be easy; its efforts to stir up fear and hatred upon which it can feed usually go unnoticed, except by those like Keyser or Walter who are sensitive to its presence. Keyser tells the Doctor that Walter will probably try to go after Harper at the next rally, but when he realizes that the Doctor intends to stop Walter he attacks him, insisting that Harper is a monster and must be stopped. Ace and a security guard pry Keyser away from the Doctor, and the Doctor and Ace depart, no closer to a solution. If Walter kills Harper at the rally the monster will just move on to a new host, and all Walter will have done is commit murder...

Harper's PR consultant, Roderick Allingham, warns her of the Doctor's involvement; he used to work for the Ministry, and knows the Doctor to be a freelance consultant to the UN. A UFO nut like Keyser would no doubt also have known of the Doctor and his involvement with UNIT, and the Doctor's attempt to make contact with Walter Jacobs suggests that the UN may be backing their attempt to destroy the New Brittania party. Allingham will therefore arrange for the police to add the Doctor to their wanted-for-questioning list. Meanwhile, preparations for the rally proceed as planned, and extra security precautions are put in place. As Harper prepares to take the stage, Allingham assures her that nobody can get into the rally without their knowing it -- but unsurprisingly, he's failed to take into account the TARDIS' ability to materialize out of thin air. The Doctor and Ace have deduced that Walter arrived before the security guards and has been waiting for Harper to take the stage. As they search for him Ace hears just enough of Harper's speech to realize that she's urging God-fearing white folk to speak up against those who would destroy their traditional values. The Doctor is more interested in the beat of the speech than in the actual words -- Harper's rhetoric has been cleverly calculated to get the crowd feeling, instead of thinking. And what they are feeling is fear.

Ace manages to restrain Walter while the Doctor defuses the bomb. Walter collapses, in a state of shock, and Ace takes him out to the street while the Doctor investigates the sound of gunfire. He helps to organise the safe evacuation of the hall and learns from the flustered Allingham that the balaclava-clad attackers fled without harming anyone -- but Allingham then realizes who he is, and the Doctor is forced to flee before Allingham can have him arrested. After moving the TARDIS he meets with Ace and Walter, and plans to investigate the attackers and find out what their agenda is. Walter, however, insists that they must first stop the monstrous Harper, and the Doctor decides to take him to Paul Tanner's flat to relax and try to convince Paul to infiltrate Harper's campaign. Meanwhile, Harper, apparently shaken by her near escape, leaves it up to Allingham to find out what the Doctor is up to and why the UN seems to want her dead.

Paul is somewhat taken aback when the Doctor and Ace burst into his flat, carrying a would-be assassin on the verge of a nervous breakdown. The Doctor switches on Paul's radio, and hears Mick Thompson playing a recorded message from Alexsandr Karadjic, the leader of the United Front terrorists. Speaking through a vocal synthesiser -- perhaps -- Karadjic tells the world that the United Front cannot tolerate Harper's ethnic cleansing agenda; they are therefore engaging in a series of surgical strikes against the New Brittania party. It appears that the Fearmonger is drawing unstable people to attack its host, causing further panic. Harper makes a statement deploring the violence, and the Doctor is forced to switch off the radio when Walter hears the monster in her voice and panics. He and Ace manage to calm Walter down, and as he falls into an uneasy sleep they explain to the disbelieving Paul that there really is a monster out there. They are not sure of its origins, but legends claim that a number of psychic forces were engineered by an alien race to incite group emotions and unite their people; when their civilisation collapsed the Fearmonger escaped to Earth, where it has been stirring up fear ever since and feeding upon the results. Paul is frankly skeptical until the Doctor builds a force field generator out of the spare parts in his junk cupboard -- a field which he can tune to the Fearmonger's wavelength, crushing it out of existence without harming its host. But now that he and Ace are known to Harper's people, they can't get close enough to use it... which is where Paul comes in.

The Doctor and Ace take Paul to New Brittania party headquarters, where he opens a window to let them in and then tries to keep Allingham distracted while the Doctor and Ace set up the machine in Harper's office. He is too frightened by what he's involved with to be convincing, however, and Allingham soon gets him to admit that the Doctor and Ace are breaking in. Allingham alerts security but gives the Doctor and Ace time to escape, telling Paul to lead them back to his flat and keep them busy so the police can catch them with Walter. On the way back, Paul tries to defend Harper's political stance, claiming that she has good points to make even if she does go over the top with them; but the Doctor points out that he only makes it easier for her by refusing to listen to the things he doesn't want to hear her saying. They return to Paul's flat only to find a crackling blob of electric, psychic energy hovering in mid-air -- the Fearmonger is trying to frighten them away before they can harm it. If it so chooses it can release a charged of its stored emotional energy into them, condemning them to a lifetime of fear, just as it did to Steven Keyser. The Doctor manages to earth it with a frying pan, and it retreats, badly drained -- and Paul, terrified by what he has seen, admits that he betrayed them and urges them to escape. When Allingham and the police arrive Paul claims that they overpowered him and got away, but Allingham reminds him that he's been conspiring with known fugitives -- and if they should try to contact him again, he'd better remember that he's not in the clear yet.

Walter realizes that the Doctor intends to leave Harper alone once he's dealt with the Fearmonger, and in his despair he tries to leap from a bridge. Ace stops him, understanding that he's terrified of what will happen if Harper gets into power, and convinces him that there's a peaceful way to stop her. The Doctor admits that it would be easy for him to stop Harper himself, but he's interfered in Earth's history far too much already -- and he knows from experience that people will still muddle through without his help. But before he has decided upon his next move, the United Front bombs a New Brittania regional office; nobody is killed, but the Doctor knows he'll have to do something before they go too far. Walter reluctantly uses his contacts to arrange a meeting with the United Front, although he isn't convinced they should be stopped. Upon arriving at the rendezvous, however, the Doctor, Ace and Walter are confronted by masked terrorists, blindfolded and shoved into a van. Their encounter is witnessed by the police, however, and word eventually reaches Allingham that the Doctor has been seen consorting with the United Front. Even if he manages to get away from the Front in one piece, Allingham has enough evidence to hold him as a suspected international terrorist until after the election...

Ace is going into shock, the Doctor is horrified, and Walter is stunned; this isn't what he wanted at all, and he refuses to join the Front unless Karadjic agrees to leave the Doctor and Ace alone and let Walter call an ambulance. Karadjic hesitates but eventually agrees, and they depart as the Doctor tries to staunch the flow of blood from Ace's shoulder. The ambulance takes Ace to the hospital, where the Doctor calls Paul and has him tell Allingham what has happened; Paul thus clears his name, and when he claims that he's lost his job due to "competition", Allingham agrees to get him a position with Harper's campaign. Before turning the Doctor over to the police, Allingham tries to get a statement himself, but the Doctor refuses to tell him anything about the Front and insists that he's just trying to stop people from getting hurt. The Doctor's UN pass allows him to get through the police interrogation with a minimum of effort, and he returns to the hospital to wait for Ace to awaken. While there, he is confronted by the Fearmonger, but it can do nothing to him; the one thing he's really afraid of has already happened.

Walter doesn't understand why Karadjic is being so accommodating to him, until Karadjic explains that it was Walter's assassination attempt that inspired him and his friends to form the Front. He and Walter set off to firebomb Harper's house, only to find the Doctor there, having tried to get in to speak to Harper. The Doctor, surprised to realize that Karadjic already knew Harper wouldn't be home, tries to warn Walter that he isn't frightening people away from Harper; he's just frightening them, and is doing the Fearmonger's work for it. Walter, furious, throws his unlit Molotov at the Doctor to shut him up, and is thus unable to destroy the house; but he and Karadjic depart without listening to the Doctor's attempts to talk sense into them, and the Doctor later learns that the United Front has bombed four other party candidates' homes. All he can do now is wait for Ace to recover and try to stop the United Front from making things worse while he awaits his next chance to tackle the Fearmonger.

Over the next three weeks, the United Front continues its campaign of terror and the political climate grows ever more fearful. The Doctor finally gets another chance at the Fearmonger when Harper agrees to make an appearance on Mick Thompson's show. Paul installs the Doctor's force field generator in Thompson's booth, claiming that it is a security precaution -- but as another security precaution, Harper arrives an hour earlier than she had claimed. The Doctor, unaware that time is already running out, is reluctant to leave Ace alone in the hospital, but she knows Paul can't handle things on his own and assures the Doctor that she will be all right. But soon after the Doctor departs, Walter shows up, wanting to apologise for what the United Front did to Ace, but still afraid of the Doctor. He believes that the Doctor has been running covert ops for the United Nations since the seventies, and when Ace laughs at him he is furious and threatens her. Trying to calm him down, she insists that he turn on the radio to listen to the Doctor crush the Fearmonger out of Harper... but they are surprised to hear that Harper is already on the air, and that Walter can no longer hear the monster -- until Thompson speaks... Ace still can't hear anything untoward until the frantic Walter threatens her further, and then she realizes he's right. The sound of the monster is in Thompson's voice...

The Doctor arrives at the studio with only minutes to spare, and manages to collapse the force field around Harper -- but nothing happens, and he can't make sense of the readings his equipment is giving him. Walter calls the show himself and manages to warn the Doctor that the monster is in Mick now, but the Doctor and Paul are unable to redirect the force field to surround Mick. Walter tries to keep Harper talking while the Doctor works, but she's far more capable of defending herself with rhetoric than Walter; he may be brave when he's got his guns and his bombs, but when he's alone he can't even talk to her. Before Walter can stop her, Ace grabs the phone from him and lays into Harper and Thompson, claiming that Harper only came out of hiding for this show because she knew Thompson wouldn't ask her any really challenging questions. He's just trying to get people to react, not think. Thompson, however, claims to be an entertainer, and that only a moron would take him seriously or get offended by his jokes. He cuts her off before she can defend herself, and she is furious, realizing that she let herself get drawn into his game and has only added to the noise. And what's worse, she's just broadcast live the fact that Walter is here with her. She is still too weak to get away by herself, and is forced to flee with Walter, moments before the police arrive. As they leave, however, they see people in the street throwing stones at each other, and realize that riots are breaking out.

Ace is in no position to do anything about the monster in the Doctor's voice, particularly not with the mob on its way. The Doctor has a plan to stop the riot, although Ace believes he just wants to prevent her from becoming suspicious of him. He offers Thompson an exclusive interview with those responsible for the rioting, and Thompson, realizing that he won't be able to get to safety in time, reluctantly agrees to call his producers and arrange to broadcast live via his cell phone. Walter now believes himself to be crazy, but the Doctor and Ace convince him that only he can help them save people's lives, and he reluctantly gives them the United Front's secret code words and meeting places. The Doctor has Ace call Paul and arrange for him to make a few phone calls... but while doing so, Ace also tells Paul to fetch something from the Mile End ampitheatre, which she will need to use later. She returns Thompson's phone, and they make a break for his car -- but they've left it a bit too long, and the mob has trashed both his car and Walter's. They will have to make their way through the mob on foot. Before leaving, the Doctor climbs atop a ruined car, distracts the angry rioters by juggling, and tells everyone within earshot to tune in to Thompson's live broadcast if they want to find out who's really responsible for the riots. Hopefully, people will listen...

Allingham is hard at work ensuring that the media coverage of the riots is favourable to New Brittania, and that party candidates are being filmed making public statements deploring the violence. The ads will be on air within the week, and the fact that Ace and Walter fled the hospital just before the riots broke out plays into their hands perfectly. Harper, however, is somewhat taken aback by Allingham's unfortunate choice of words, when he suggests that the focus on the conspiracy will help ensure that New Brittania doesn't seem responsible... almost as if they actually are. The police then receive a call, apparently from a United Front spokesperson, deploring the indiscriminate violence and calling upon New Brittania party leaders to meet with them before things get wholly out of hand. Allingham is surprised to learn that the message has been confirmed as genuine, and despite the danger posed by the mobs, he prepares to set off for the meeting. He advises Harper to remain here, where it's safe; he may not believe what he says in public, but he appreciates the rarity of a politician who does. Harper, upset that her intended bloodless revolution has been tainted by these riots, remains at party headquarters, waiting for him to return.

As the Doctor, Ace, Walter and Thompson make their way through the riots, they see people being dragged from their homes and beaten, and Thompson, surrounded by angry and frightened rioters, is forced to step over a woman he sees bleeding in the middle of the street and continue on his way. Walter is in shock; this isn't what he'd wanted at all. The Doctor is struck by a flying brick and momentarily knocked out, but when he recovers Ace suspects that he pretended to lose consciousness to make her afraid for him. Thompson is unable to calm down the mob, who are too angry to listen to him. They eventually reach their destination, a safe house where Karadjic is sheltering from the mobs. He is infuriated when he realizes that Walter has brought the Doctor, Ace and Thompson, but the Doctor points out that the mob outside is afraid and wants a scapegoat -- and that Karadjic's only choice is to let Thompson interview him and tell his side of the story.

Karadjic admits that there is no organisation behind the United Front -- it's just him and some like-minded friends who got together to take out their frustrations on New Brittania. Allingham then arrives, calling out to Karadjic by name and telling him that it's all under control, and realizes too late that the Doctor is present. It was Allingham who supplied the United Front with government-issue weapons, seeing them as a ready-made publicity stunt; Karadjic may have thought he was using Allingham to frighten people away from the party, but Allingham is just as convinced that he's been using Karadjic to frighten people into supporting Harper. Now, however, the fear they've created has gotten out of control and is feeding upon itself. The Doctor, satisfied, reveals that they've just broadcast their confession live on Thompson's show; it was Paul who called the police to set up this meeting, using the codes which Walter had given to the Doctor and Ace. Allingham claims that Harper was unaware of any such deal, and refuses to say anything else until he has spoken with his solicitor.

The police arrive and arrest Allingham and Karadjic, while Walter is taken to an institution for treatment; Ace assures him that he'll be well looked after, and that she will deal with the monster herself. She returns to the warehouse, claiming that she's going to collect the force field generator, while the Doctor stops off at New Brittania party headquarters to speak with Harper. The riots are dying down, but the mob is still on its way to New Brittania; nobody believed Allingham's claim that Harper was innocent, as she's spent so long telling the people not to trust anybody. Harper admits that she too was involved in Allingham's plans to use the United Front as publicity, but insists that she's not responsible for the rioting; she just wanted to make people afraid so they would turn to her. And now they understand who has been making them afraid -- and they're coming for her. The Doctor departs, leaving Harper as she always has been -- terrified, and alone.

The Doctor then returns to collect Ace, but she flees from him and shuts herself behind the force field so he can't get at her. On her instructions, Paul has collected the bomb which Walter was going to use in the Mile End ampitheatre, and set it up next to the force field generator -- and if the Doctor doesn't allow Ace to use the force field on him, she will set off the bomb. She won't let the Fearmonger turn the Doctor into a vessel of hatred and fear; she's willing to die to save him from that. Deeply saddened, the Doctor tells her to listen to him -- not to the noise in his voice, but the meaning of his words. It's not he who is full of fear and rage, and who is ready to kill to make the monster go away... just as it was never the speeches or the politics that spread fear throughout the population, but the men with the bombs and guns. The Fearmonger was never in Harper or Thompson -- it was in Keyser and then Walter, making them believe that the monsters were out there, driving them to kill and feeding off their fear. And when Walter threatened Ace while she was alone and frightened in the hospital, it passed into a new host once again. The monster isn't in the Doctor, but in Ace. Ace tries not to listen, but can't help it, and realizes that he is telling the truth... and she closes up the force field around herself, crushing the Fearmonger out of existence.