The Doctor offers to take Ace and Benny to Key West in 1915 for a relaxing vacation, but instead the TARDIS materialises in Haiti. General Bobo’s rebels are preparing to attack the capital and depose the corrupt President Sam, while Admiral Caperton’s Marines wait off the coast of Cuba for the opportunity to move in and seize control. The Doctor and his companions, still trying to determine exactly where they are, stumble across the bodies of rebels killed by the Secte Rouge and are found standing over the bodies by Captain Eugene Petion. Petion confiscates the TARDIS key and takes his prisoners to General Etienne for questioning, but the Doctor produces UNIT documents which seem to prove that he and his friends are British secret agents. Assuming that they are here to investigate the recent disappearance of a British envoy, Etienne reluctantly sets them free, but has the TARDIS brought to the palace for closer examination and orders Petion to keep an eye on them. In any case, the Doctor intends to remain, as he senses an odd telepathic whisper at the back of his mind, and believes that something odd is going on nearby.

Elsewhere on the island, in a secret underground base, German scientists Karnstein and von Stein are conducting research, although Karnstein objects to the direction their work is taking. Security is provided by Paul Reichmann, an American mercenary whose only skill is murder. The Germans are in league with a vodoun bocor named Henri, who himself reports to a houngan named Lemaitre, or Mait. Henri’s followers recently killed the British envoy, who had discovered evidence of the German presence on the island and was about to report it to his superiors. Henri draws Tarot cards to predict the outcome of their operation, and has a vision of three newcomers -- a man and two women who bring disruption and chaos in their wake. Mait orders Henri and Reichmann to find out if the newcomers pose a threat, and to dispose of them if necessary. Reichmann agrees reluctantly, as he does not believe in Henri’s fortune telling and does not appreciate having to take orders from him.

Mait sends a warning to Sam, telling him that his soldiers are deserting to join Bobo’s forces. Realising that the people are turning against him, Sam arranges to have most of his country’s gold standard loaded aboard an escape boat so he can flee to a luxurious retirement. But Mait is also helping Bobo, and he casts a coup l’aire on the boat, killing anybody who steps aboard. Furious and frightened, Sam orders Etienne to make an example of anyone who has dared to speak against his rule by executing everybody in the prison -- an order which the sadistic Etienne is only too happy to carry out. As news of the slaughter gets out, the people’s mood turns ugly, and the horrified Petion decides that he can no longer follow Etienne’s orders with a clear conscience.

The Doctor senses more telepathic whispers from the docks, and Petion leads him and his friends to the French compound, where they find the bodies of two men who appear to have been torn apart by people using their bare hands. One of the men was interrupted while writing a journal, and the Doctor learns that the Frenchmen were under attack from their fellows -- men who had vanished some time ago, only to return as animated corpses who kept attacking even after being shot through the chest. Petion takes the Doctor and his companions to the local university hospital, where an American doctor, Howard Philips, has been researching the medical history behind the legends of vodoun. Once the Doctor convinces Philips that he is not a sensationalist reporter, Philips shows him an unidentified white man whose body was recently washed up on shore -- and who returned to life after his autopsy and nearly killed two morgue attendants. He has been chained up ever since, lashing out in blind rage at anybody who comes near him. This is not an ordinary tetrodotoxin victim, one of the vacuous, easily controlled zombis of the island; this is something entirely new. Meanwhile, elsewhere on the island, the Germans continue testing the chemicals which Mait has supplied to them -- chemicals which slow the rate of blood flow in the human body, and make it more resistant to gunshot wounds.

The Doctor notices a set of carvings in Phillips’ office, which Phillips claims were found at around the same time that the “dead” man was washed ashore. It may just be a coincidence, but the carvings appear to depict the Old Ones, horrific creatures which inhabited the Earth long before mankind evolved and which were forced by a cosmic disaster to flee into the Vortex as discorporate entities, leaving their bodies behind. They have sought to return ever since. The Doctor sends Ace and Petion to collect the TARDIS from the palace while he and Phillips set off to examine the area where the carvings were found. Benny, meanwhile, visits the museum to study more of the carvings, but Henri’s associate Carrefour spots her and gets to the museum first. He dusts the carvings with a coup poudre, and when Benny touches them, she absorbs the chemicals through her skin and falls into a coma like living death. Carrefour has her body taken to the German base for interrogation, while Mait -- concerned about the threat posed by the visitors -- orders Bobo to make his move at once.

Upon returning to the palace, Petion is nearly arrested for desertion, but Ace overpowers the poorly trained guards and takes Etienne hostage, forcing him to tell her where the TARDIS has been stored. Fortunately, it’s in the armoury, and Ace locks herself and Petion in with it and stocks up on weapons while the furious Etienne tries to break in. He does so -- just in time to see the TARDIS vanish into thin air as Ace pilots the amazed Petion back to the hospital. The Doctor and Phillips, meanwhile, find evidence that the carvings were unearthed by some sort of underground activity, but before they can investigate further, Bobo’s rebels attack Port-au-Prince and fighting breaks out in the streets. The Doctor and Phillips try to return to the hospital, but are attacked on the way by Reichmann and two German soldiers. Reichmann, already angry with Henri and Mait for advancing the schedule because of their superstitious fear of the newcomers, is even more infuriated when the Doctor escapes from him. The Doctor returns to the hospital with Phillips, and finds that Ace has used the weapons from the armoury to prepare for a siege. The Doctor tells her that she’ll only have to hold out until morning, when the Marines will land.

Benny recovers from her exposure to the zombi powder and escapes from the German medical bay. The base has been built within ancient natural caverns -- at least, they appear to be natural, although some of the the angles appear wrong in ways Benny can’t quite grasp. She eventually finds her way to a vast cavern where German boats, U-boats and even a dirigible are stored, along with crates containing what appear to be chemical weapons. Benny is unable to read the writing and thus transcribes it instead, but is then spotted by guards and must swim through underwater tunnels to escape. By the time she gets to safety, the Marines have landed, and some of them take Benny back to Port-au-Prince while others remain to investigate a nearby village where all of the inhabitants seem to have died for no apparent reason.

Sam commits suicide when Bobo and his men attack the palace, and the fleeing Etienne heads for the Doctor’s hotel room to seek revenge -- only to be killed by a poisonous snake planted by Carrefour. As the Marines seize control of the capital, the Doctor has Petion take him to a houngan of the Rada Loa to investigate Benny’s disappearance. The houngan, Dubois, is also an Empereur of the Bizango, the unofficial rulers of the village communities, and as such he has heard rumours of Mait’s evil activities for some time. He has also heard that Mait intends to conduct a Wede Man No Dlo ceremony in a nearby cemetery tonight, and the Doctor decides to enlist the Marines’ aid to stop it. Colonel Mortimer has orders to put an end to the natives’ “black magic nonsense”, and with a little prompting from the Doctor’s hypnotic abilities, he agrees to take his men to the cemetery to intervene.

The Doctor takes Ace, Petion and Dubois to Mait’s villa, where he finds a mind mirror capable of amplifying telepathic communication. It is this which Mait has been using to control his new, killer zombis; the attack on the French compound was a test of his abilities. Its design must have been inspired by a dream sent to Mait by the Old Ones; nevertheless, it is still primitively constructed, and the psychic fallout from its use has contaminated the island with the whispering which the Doctor sensed earlier. When the Doctor was attacked on his way back to the hospital, the men were armed with German Lugers, and the Germans were amongst the first to use chemical weapons in warfare; therefore, Mait must be supplying them the chemicals used in the vodoun rituals to create zombis. If used in quantity in the European war, the chemicals will transform the population of Europe into mind-controlled slaves… and open the way for the Old Ones to return to Earth unopposed.

The Doctor has Ace blow up Mait’s storehouse while he resets the mind mirror to entrap Mait’s spirit should he use it again. He then sends Ace back to the hospital to organise the Marines’ attack on the cemetery, while he tries to desecrate the cemetery in order to delay the ritual. Mait, however, learns of the attack on his villa and returns with Henri, who frees him when Mait looks into the sabotaged mirror. Mait, shaken, resets the mirror and gazes into it again, this time picking up a telepathic impression of the last person to use it -- a Time Lord who calls himself “Doctor”. Realising what the Doctor intends to do, Mait sends Reichmann to the cemetary to stop him, but the Doctor convinces Reichmann that Mait intends to betray the Germans and has already planted a spy in their base. Reichmann takes the Doctor back to the base to explain himself to General Froebe, but Mait intercepts the report and confronts the Doctor personally. The Doctor is somewhat disappointed to learn that Lemaitre is not the man he was expecting when he first heard the name; instead, he’s just another human maniac, whose life was extended by the Old Ones and who has spent the last two hundred years plotting to destroy the blancs for the slave trade. The Doctor reminds Mait that his own tribe, the Efik, participated enthusiastically in the trade until they came under quota and were taken by the men whom they had supplied with their enemies. Mait storms out, infuriated, and the Doctor hypnotises his guard and escapes; he then locates the armoury and begins planting explosives about the base.

Benny returns to the hospital with evidence that a German naval base is located nearby, and when her notes are translated they prove that the Germans are intending to ship mass quantities of the zombi chemicals to Europe. Petion tells Ace and Benny that the Wede Man No Dlo ceremony is intended to reunite the discorporate soul with the body -- which suggests that if Mait completes the ritual successfully, he will restore one of the Old Ones to life. The Marines head for the cemetery, but Mortimer is unwilling to attack until the most opportune moment presents itself -- and thus, by the time he does actually launch his attack, Mait’s mind mirror has already stored enough psionic energy to be of use. Mait, Henri, Carrefour and Reichmann escape back to the naval base with the mind mirror, closely pursued by Ace, Benny, Petion and the Marines. Ace uses a hand laser to cut through the blast doors, and the Marines storm the German base.

In the confusion which follows, Reichmann finally tires of taking orders from Henri and shoots him. Mait, meanwhile, tries to get the mind mirror to the Old One’s resting place to reawaken his master, and the Doctor pursues him, telling Ace to keep the Germans occupied until it is too late for them to defuse the explosives he has set. Benny tries to take Froebe prisoner, but finds Reichmann and Carrefour waiting for her. When Reichmann attacks her, provoked by her taunting, Carrefour is reminded of the men who killed his own mother, and spontaneously attacks Reichmann. Reichmann accidentally shoots and kills Froebe as they struggle, but then kills Carrefour and pursues the fleeing Benny. He encounters Ace instead, but Petion shouts out a warning to her -- and when Reichmann shoots Petion, the infuriated Ace empties her own gun into the startled Reichmann in a fit of rage. Mortimer is killed in the fighting, while Karnstein -- who had agreed to conduct research into the zombi chemicals when it seemed they would be used to save lives, and was horrified by the thought that they would be used to rob people of their free will -- takes the opportunity to torch the laboratory, destroying her notes.

The Doctor uses Ace’s hand laser to cut through the tunnels and reach the Old One’s resting place first. There, he finds a gigantic mind mirror, which he sabotages just in case anyone ever manages to get through to it again. He then returns along the tunnels, planting motion-sensitive explosives behind him. When he encounters Lemaitre, he tries to talk sense into him, but Mait is obsessed with revenge and continues on, thus triggering the explosives which the Doctor had planted. Mait is killed and his mind mirror is destroyed. The Doctor returns to the main cavern and warns everybody to evacuate, which they do, just as the explosives he had planted earlier go off, destroying the German base and their chemical stores.

The only victims of the German chemicals are the inhabitants of the unnamed village which was used as a test site, and they too will recover eventually. Petion will lose his arm but will survive, and the Old Ones remain drifting aimlessly in the Vortex, waiting for a chance to try again. But as the Doctor and his companions return to the TARDIS, Ace finds herself unable to shake the image of Reichmann’s death from her head. All she’s done for the past three years is kill in the service of Spacefleet, and it seems that she’s forgotten how to do anything else. Is she starting to become just like Reichmann?