It is late at night, past closing at the Museum of Aural Antiquities. The blind curator, Gantman, plays some of the Museum's new additions -- sound files from the repertoire of actor Visteen Krane -- and listens to research student Miles Napton praising Krane's work. Gantman critiques Krane's delivery of the closing speech of "The Good Soldiers", but agrees with Napton's assessment of Krane as the finest actor of their age. Were it not for his tragic death Krane would no doubt have been elected President, and now there is nobody in the race worth voting for, except possibly Beth Pernell. As Gantman prepares for bed, Napton tells him not to bother deactivating the security systems, as he intends to remain in the museum and work through the night. But two unauthorised visitors have just broken into the museum; Amber Dent and Goff Fotherill, armed with software patches and a list of alterations, are here to digitally alter Krane's recorded speeches. As they prepare to sample Krane's voice and begin their work, Fotherill hears somebody in the sound galleries whistling the "Death March", and Dent sends him to ensure that the whistler is leaving. The galleries appear empty and silent, apart from an odd whispering echo, but as Fotherill prepares to return to the editing suite he hears Dent calling to him from next to the security archive door. The voice, however, is not Dent's -- and the whispering echo suddenly rises to a tortuous volume, causing the agonised Fotherill to stagger helplessly into an electrified security door...

The TARDIS materializes in the sound galleries, and the Doctor sets off to explore, followed by a rather less enthusiastic Peri who thinks she hears whispering in the distance. They soon determine that they are in an archive dedicated to the preservation of recorded sound, and as they explore further they happen across the suite where Dent is playing back one of Krane's speeches -- "no single man is greater than his policies". Dent hurriedly erases the computer change log and flees, and the Doctor, realizing that she was up to no good, sets off in pursuit. He and Peri lose Dent in the galleries, but stumble across Fotherill's body. The Doctor soon determines how he died, but it is not clear whether the dead man stumbled accidentally into the door, or was pushed. Officer Berkeley, called in by the museum's automatic security system, arrives and catches the Doctor and Peri -- two strangers standing over a dead body next to the electrified security archive door, which protects the security service's highly classified recordings of interrogations, wiretaps and surveillance tapes. Although suspicious, he is unsure whether these remarkably conspicuous characters could possibly be attempting something surreptitious; but whether or not they are guilty of murder, they are definitely trespassing and must explain themselves to Gantman. Meanwhile, Dent hides in the galleries and tries to contact Fotherill, but picks up only a distant whispering sound on her communicator...

Gantman confirms that the Doctor and Peri are unauthorised visitors; only he and Miles Napton should be in the museum at this time of night. The Doctor and Peri describe their encounter with the young woman, but when Peri quotes from the speech she heard -- "no single man is greater than his policies" -- Gantman gently corrects her; he entered the speech into the archives only the other day, and it actually runs "a single man is greater than his policies". The Doctor requests that the speech be replayed to prove Peri's honesty, in order to establish their trustworthy credentials, and Gantman is shocked when Peri proves to have been right. Having lost his sense of sight, sound is all he has left, and he can't believe he would remember this speech incorrectly. Unfortunately, Visteen Krane was a very private man who shunned the visual media, and this particular speech is not in the public domain; thus, this is the only version in existence, and there is no way to verify Gantman's claim that it has been changed. The Doctor has become intrigued, and in order to stay close to the TARDIS he insists upon helping Berkeley to solve the mystery. Krane's agent Beth Pernell and producer Hans Stengard are soon to arrive at the museum to prepare a broadcast of Krane's favourite speeches as a tribute to the late actor; perhaps Pernell will be able to recall the correct wording.

Peri tries to follow Dent, but loses her in the museum's corridors. Pernell and Stengard arrive to supervise the tribute broadcast; Stengard is here to liaise with the radio station and handle the technical side of things, while Pernell is here to represent Krane's interests; she is his fomer agent and would most likely have been his running mate if he had not died before making his official announcement. Pernell is surprised to find two strangers and a security officer at the museum investigating a murder, but agrees to listen to the suspect speech; however, she claims to be uncertain whether it has been altered. Stengard suggests checking the computer's security logs, tsking sadly when he realizes this obvious step has been neglected. According to the logs, the last file to be retrieved was the one which Gantman played for Napton earlier -- but when Gantman replays it to confirm this, he is shocked to find that it too has been changed, almost as if his earlier comments on the delivery had been taken into account. While Stengard sets off to set up the equipment for the broadcast, Gantman searches for Napton and finds him in Krane's sound suite; Napton claims to be all right but requests that he not be disturbed, as he is close to a breakthrough in his research.

The Doctor wonders why someone would risk a criminal record just to change a few political speeches, and inquires whether Pernell intends to run in the election. Pernell is so infuriated by the Doctor's tactlessness that she slips up and admits that she does indeed intend to nominate herself in Krane's place -- in fact, she intends to do so after the broadcast of his final speech, the one he was rehearsing when he died, in which he was to have named her as his running mate. Meanwhile, Stengard runs into Dent, who begs him to save her from the voices haunting her, voices which will never leave her alone until she confesses what she has done. Stengard turns her in, and she is locked up in one of the sound suites -- somewhere soundproof, at her own request. Berkeley thinks she's been driven mad with guilt, but the Doctor recalls the odd sounds Peri heard when she pursued Dent -- sounds with no apparent source -- and theorizes that, somehow, the Museum of Aural Antiquities has given birth to a sentient sound wave, a creature capable of modulating itself as any sound whatsoever. Until they determine the creature's origin and agenda, they must seal the museum off completely; such a creature could potentially escape via any medium capable of transmitting sound waves...

The creature modulates itself to copy Gantman's voice, thus gaining access to the museum's search engines and editing equipment. The conversation between Fotherill and the unknown voice is replayed, and at the creature's command the computer alters the sound of the voice, changing its pitch and tone, removing the background noise, and deconstructing the pattern of the voice synthesiser. When its analysis is complete, the voice is revealed to be that of Beth Pernell -- and further searching locates a recorded conversation between Pernell and Stengard, in which Pernell admits that she believes Krane's intentions are not in her best interests... Meanwhile, Pernell and Stengard discuss the changes to Krane's "Good Soldiers" speech, which was not one of the files on the list. Something has gone wrong, and now that they're this close to success they cannot permit anything to get in the way -- particularly not the unstable Dent. Stengard thus "visits" Dent in her sound suite and stabs her to death, tsking sadly; how unfortunate it was that Berkeley did not find the knife when he searched her, and how screwed up she must have been, to let the voices drive her to this... He reports success to Pernell, who is certain that they cannot fail now; soon there will be a better world for everyone. She ignores what appear to be distant echoes in the galleries -- quiet whispering, like the sound of insane laughter...

Even when alive, Krane was emotionally overwrought and fanatically possessive about his work -- in fact, he shot himself in the head while talking to Pernell from his own sound suite. And now he has become a killer capable of hiding himself within any sound, however quiet; the hint of a distant conversation, the soft hum of the air conditioning... The only way to destroy what he has become is to generate a sound wave with an opposite modulation, thus cancelling him out, but in order to do so they must know exactly how he has modulated himself. The Doctor and Peri return to Krane's sound suite, hoping to catch the creature in the act of altering another of the speeches. The computer detects such a change occurring and alerts the Doctor, who turns down the audio output, accesses the file and moves it to a backup storage facility. The creature is unable to escape in time, and the Doctor shuts down the sound suite, satisfied that the killer is now stored as a backup file on a holographic disc -- where it will remain as long as the disc is never played.

Pernell and Stengard, however, need to access and interrogate the creature, to find out what files it changed while it was free; it may have restored Krane's original speeches, or faked their confessions to crimes real or invented. Stengard offers to seal the disc in a secure package, but the Doctor and Berkeley feel it would be safer in their possession. Stengard thus "accidentally" bumps into the Doctor and switches the disc with a copy, tsking sadly as he apologises for his clumsiness. While the grateful Gantman leads the Doctor on a tour of the museum, Pernell and Stengard find an empty sound suite, disconnect the audio output and order the computer to reproduce a simulation of the waveform playing on the disc -- thus hearing, not the sound itself, but a copy of it. Pernell then attempts to interrogate the creature, demanding to know what it has changed -- but it is furious with her for altering its work, and vows to stop her, whatever the cost. When it refuses to co-operate, Pernell begins editing the source waveform on the disc, deleting sections from the middle, stretching others, rearranging and adding reverberation echoes -- a process which puts the Krane creature through sheer agony...

Stengard, sickened by the sadism Pernell is displaying, leaves her to get on with it alone, but Peri -- who has slipped away from Gantman's tour to check on the TARDIS -- hears the door closing and investigates. She thus finds Pernell torturing the sound creature as it reproduces conversations between Krane and Pernell -- conversations which make it clear that Krane was disgusted with Pernell's ambitions and medieval political idealogy. Peri flees, runs into Stengard, and leads him back to the sound suite to show him what Pernell is doing -- and realizes too late that they are in league. Stengard tsks sadly as Pernell continues to torture the creature, which replays Krane and Pernell's final phone conversation. It appears that Pernell is trying to talk Krane out of some decision he has made -- but he tells her that there is a gun at his head, and after a quiet ticking sound, the gun fires. Pernell and Stengard must now deal with Peri, but she lunges at the controls and turns the volume full up as a distraction. She thus manages to escape -- but the creature too flees, free to kill again. Pernell, rattled, nevertheless orders Stengard to return to the broadcast suite and prepare to link with the studio -- but Stengard, hearing the creature's cries as it flees into the depths of the museum, knows that Pernell's torture has driven it insane...

With only seconds to go, the Doctor appeals to Krane's vanity; if he allows the broadcast to go ahead, it will appear that he is endorsing Pernell -- and, thanks to him, she will win the election. The creature draws back and allows the Doctor to get to the broadcast suite, but when he begins to operate the controls Pernell holds him at gunpoint. Rather than try to stop the broadcast, however, the Doctor has switched on the intercom, allowing Peri, Gantman and Berkeley to hear him and Pernell. As the radio announcer introduces the show and builds up to the broadcast of Krane's speech, Pernell assures the Doctor that the speech will be very convincing. He reminds her, however -- and thus Peri, Gantman and Berkeley -- that the speech has been in the archives for some time; he could already have heard it, and might even be able to speak along with it if he wished. Peri is the first to realize what he is trying to tell them; since they already know what form the creature will take, they can create a wave form with the opposite modulation, silencing the broadcast -- and destroying the creature hiding within. Gantman sets to work and successfully sets up a cancelling wave, moments before the speech begins; thus, although Pernell hears the speech in the broadcast suite complete with Krane's announcement that he and Beth Pernell can work together to make their world a better place, nothing is actually broadcast.

The Doctor overpowers Pernell and escapes, locking her in the sound suite. Stengard then informs her of the problem, and tries to solve it while the studio announcers cover the silence. The Doctor, meanwhile, gets back to the control suite and contacts Pernell, trying to keep her talking while Gantman seeks a more permanent way to block the broadcast. The Doctor insists that if elected, she has a responsibility to stay true to the promises she made to get herself into power, but she scoffs at this concept; as far as she is concerned, democracy is an outdated system which disperses power so thinly that it barely exists, and only a firm leader such as herself can restore her world to a position of strength. The Doctor now realizes that in trying to stop Pernell, the Krane creature was closer to his morals than he had thought -- but when he tells Pernell that Krane is finally dead, Pernell switches off in triumph without listening to a further word; Krane couldn't stop her while he was alive and he certainly can't now.

Gantman wonders at Pernell's odd choice of phrasing -- and then the creature returns to them, not dead after all, and replays the recording of Krane's suicide. The Doctor listens to it closely, and after replaying it and isolating the last moments of Krane's life, he realizes the truth. Krane said that there was a gun at his head, but not that he was holding it; and the odd sound they hear before the shot is not ticking, but tsking -- the characteristic vocal mannerism of Hans Stengard. Krane was murdered to prevent him from delivering the speech in which he would have denounced Pernell's ambitions and disassociated himself from her. The only way to stop the broadcast now is to find Stengard and stop him, but Stengard is sure to be armed. Stengard, meanwhile, accesses the computer's audio help system, and it informs him by disconnecting the main power supply from the security systems and connecting it directly to the broadcast equipment, the security protocols can be bypassed. Stengard prepares to do so, but as he pulls the cable free he realizes too late that the voice speaking to him is not that of the computer -- and that the power cable is live...

Gantman tries to warn Napton of the danger posed by Stengard and Pernell, and eventually finds Napton in the broadcast suite. Napton claims to have stumbled across Stengard's body -- but the Doctor, Peri and Berkeley unexpectedly show up to find Gantman apparently talking to an empty room. The creature, caught out, changes its voice to that of Visteen Krane and admits that it was "Miles Napton" all along; but, although it was temporarily driven mad by its torture at Pernell's hands, the cancellation wave Gantman and the Doctor included in the broadcast had a healing effect on him and has restored his sanity. The Doctor thus enlists the creature's help, and returns to confront Pernell, giving her one last chance to back down. She refuses to do so, and the Doctor has no choice but to let the speech go out. Pernell listens in horror as the voice of Visteen Krane speaks, claiming to find Pernell's vision and drive utterly abhorrent -- the sound creature is changing the speech as it goes out. Pernell demands that the Doctor and the creature stop interfering; the people need a strong hand to drag them out of the mire of disorder their government's weak liberal policies have created, and such a change is worth anything. The creature replays Krane's final phone conversation, slightly altered to make it clear that he was murdered -- but Pernell insists that her way is the only way, and if that's what it takes, then it is worth the cost of a man's life. She realizes too late that her confession has just been broadcast live to millions of homes, and has no choice but to flee the museum, steps ahead of Berkeley, her political career at an end.