Shortly after the end of the Second World War, young Second Lieutenant Alistair Gordon Lethbridge-Stewart is assigned to update the British army’s maps of the Greek islands. He and Corporal Ned Clarke are transported to the island Zante by HMML 951, a minesweeper patrolling the Greek coast to ensure that the Communists in the north are aware that the British still have a presence in these waters. Upon arriving at Zante, the crew throw a reception for Mrs Demeter, the grande dame of the island, and Alistair is instantly smitten with Mrs Demeter’s beautiful grand-daughter, Persephone. He tries to put Sephie out of his mind and sets off to map the island with Clarke, but the next night, while camping in an olive grove, he and Clarke witness a strange incident: a man appears to be trampled to death by a bull, but when Alistair and Clarke draw closer they see no bull and no body.
Alistair and Clarke are then taken prisoner by men speaking German, who fortunately turn out to be Mrs Demeter’s servants; Alistair and Clarke have inadvertently trespassed on the family’s land. Mrs Demeter invites them to spend the night in her mansion to apologise for the incident, and while dining with the family -- Mrs Demeter, Persephone, and Sephie’s teenage brother Hermes -- Alistair tells them of the odd incident in the olive grove. After dinner, Sephie takes Alistair to the cliffs to see the view and more besides; however, the family knows more than they’re saying about the incident in the grove, and the next morning, Sephie’s attitude seems to have changed. Citing family difficulties, she tells Alistair that it would be best if he were to leave. As Alistair tries to articulate his feelings for her, Clarke arrives with bad news: Stalinist rebels in Albania have been attacking British naval ships, and Alistair must accompany HMML 951 to Corfu.
Some time after the ship’s departure, the crew discover that Hermes has apparently stowed away, seeking adventure. Lieutenant George Spawton orders him to stay below decks, out of the way, but as soon as Hermes and Alistair are alone, Hermes reveals that he teleported aboard the ship after it left in order to speak with Alistair. Hermes’ family are in fact powerful immortals who were once regarded as gods by the ancient Greeks. Most of their kind have passed into legend, and some treat humanity as pawns in a vast game, but Demeter, Hermes, and Persephone have retired to live as ordinary mortals, changing their names and moving on whenever it becomes necessary.
Hermes proves his claim by teleporting instantly across the room, and goes on to explain to the stunned Alistair that the powerful immortal Hades has kidnapped Sephie. He did so once before, trying to force her to become his bride; now, however, Hermes suspects that he has a different agenda, possibly connected to Sephie’s ability to control the weather. There is a portal into Hades’ extra-dimensional domain near the olive grove where Alistair and Clarke set up camp, and Hermes suspects that the man they saw killed was trying to get away from the Underworld to warn the family that Hades was up to something. Shortly after HMML 951 left Zante, Sephie visited the olive grove to investigate, but was set upon by satyrs and taken to the Underworld. Hermes fears Hades’ power too much to rescue her himself, but, bearing in mind the tale of Orpheus, he believes that Alistair’s love for Sephie may protect him if he ventures into the Underworld to rescue her.
There is another portal to the Underworld in Albania, and in order to reach it, Alistair suggests to Lt Spawton that he and Clarke scout out the land in preparation for the battle to come. His plan meets some opposition from the by-the-book Captain Percy Cole, but Spawton goes over Cole’s head to Admiral Urquhart, who approves the plan and makes some modifications of his own, including fitting a loudspeaker system to HMML 951 in order to broadcast the sounds of a false naval battle and trick the enemy troops into dividing their forces. Alistair must wait for the new moon so he can scout out the land under cover of darkness, and while waiting, he participates in a cricket match between ships -- and is upset when Hermes uses his powers to ensure that their side wins.
Two nights later, Alistair and Clarke set off on their mission under cover of the new moon. As soon as they land, Alistair suggests splitting up to cover more area; in fact, he intends to leave Clarke to his work while he heads for the portal to the Underworld. However, before they can do so, they stumble across an enemy camp of considerable size, and see a Spitfire strafe the camp, killing several soldiers. Oddly, the stricken soldiers merely drop dead, though the Spitfire’s ordnance should have torn them apart. Alistair and Clarke are then captured by an enemy officer, Colonel Nicolevic, who has them tied up and locked in the back of a van, intending to torture them for information, kill them, and send photographs of their bodies to the Royal Navy to prove that the British are invading foreign soil.
Meanwhile, Hermes transports himself to the Underworld, where he finds that Hades has gone for the moment to conduct business in the outside world. Hermes thus slips past Hades’ guards and locates Sephie, who is sealed behind a psychic force field guarded by a satyr. When the satyr wanders off briefly, Hermes speaks with his sister, who reveals that Hades wants to become god of all the Earth. In order to ensure that the human race is weak and divided and thus receptive to his rule, he intends to provoke an even more devastating world war by creating a storm to sink the British fleet; the Albanians will then be able to pick off the remainder, the Americans and Russians will enter the fray, and the war will escalate into a nuclear exchange. Sephie has the power to stop Hades, which is why he lured her into his clutches and is keeping her out of the way. Hermes assures his sister that Alistair is on his way to save her, but becomes worried when he realises that Sephie is genuinely concerned for Alistair’s safety.
When Alistair fails to arrive at the portal, Hermes tracks him down and rescues him and Clarke. Alistair glosses over Hermes’ unexpected arrival and sends Clarke on his way, claiming that Hermes has local knowledge that might come in useful. Alistair then accompanies Hermes to the portal, where they are confronted by Cerebus; fortunately, Alistair is good with dogs, and soon Cerebus is panting at his heels. He is less successful charming Charon, who refuses to ferry anyone across the Styx without payment. Alistair is attacked by sea serpents when he tries to swim the river, but Hermes manages to haggle Charon down to accepting two drachmas, and rescues Alistair on his way across. However, Hermes begins to lose his nerve when he realises that the monsters are active again because Hades has returned.
As Hermes and Alistair draw closer to Sephie, they see a “board meeting” between Hades and his agents in the outside world, and Alistair recognises Hades as “Colonel Nicolevic”. He and Hermes continue on their way, passing through a cavern of ghosts confined to the Underworld; the ghosts claw at the living, desperate for release, but Alistair disperses them by shouting at them to go away, much to Hermes’ surprise. Alistair and Hermes thus reach the central cavern, but Hermes’ nerve breaks and he flees back to the portal, leaving Alistair to fight Hades alone.
Sephie eventually realises that the force field around her is a psychic barrier maintained by her satyr guard, “Dog”. She thus flirts with Dog, tricks him into dropping the barrier to get to her, and flees -- only to run straight into Hades, who punishes Dog for his failure by dropping him into a cauldron of volcanic fire. While the other satyrs are distracted, Alistair ushers Sephie out of the cavern, but as they flee from Hades, they find themselves running through an infinitely long tunnel. Hades has twisted their perception of space in order to keep them trapped in his domain. Sephie helps Alistair to break the spell by going in a direction he knows to be the wrong way, and they thus find their way back to the portal -- but when they step through, instead of Albania, they find themselves back in the central chamber of the Underworld.
Hades mocks Alistair and Sephie for thinking they could escape him, and claims to have manipulated the human race throughout history as Machiavelli, Robespierre, Richelieu, and even Douglas Haig. Since Sephie refuses to obey him, Hades forces her to march into the volcanic cauldron. Alistair tries to offer his life in exchange for hers, but Hades scoff, as he intends to kill Alistair in any case. However, the offer delays Hades for vital seconds, allowing unexpected reinforcements to arrive. Ashamed of his cowardice, Hermes has returned to the cave of ghosts and convinced them that they are only confined to the Underworld by their own belief that they are doomed to remain here forever. Angered, the ghosts flood into the central cavern and fling Hades into the volcanic furnace before passing on to the next stage of existence.
Alistair, Sephie and Hermes flee through the portal and return to HMML 951 with Clarke. Though surprised by Sephie’s presence, the understanding Lt Spawton gives her and Alistair some time alone in the launch’s wardroom. Unfortunately, while Sephie is distracted, the immortal Hades rises from his furnace and sets the great storm in motion. Sephie realises what is happening too late to intervene; the storm has started, and must blow itself out. Only a god with the power of Zeus or Poseidon can stop it now. Hades appears astride the cliffs and addresses the crew of HMML 951, threatening to kill them all for their defiance, but Hermes uses the loudspeaker system to impersonate Poseidon and order Hades to retreat. Hades isn’t fooled for long, if at all -- but while he is distracted, Sephie contacts the real Poseidon, who rises from the seas, stills the storm with a gesture and casts Hades back to the Underworld.
Once Hades is gone, the diplomats manage to settle the disturbances in Albania. Sephie returns to the Underworld briefly to fetch water from the Lethe, the river of forgetfulness, and slips it into HMML 951’s drinking water, thus ensuring that the crew do not remember these bizarre events. Alistair and Sephie then spend a blissful two-week vacation in the Blessed Isles, but Sephie knows that their relationship can’t last; Alistair is only 21, and Sephie is 3,701 years old. When their brief time is over, the waters of the Lethe wait for Alistair as well.
A wave of violence is sweeping across Britain, and as there appears to be no reason or pattern to the incidents, the file ends up at UNIT. Yates convinces the Brigadier to let him and Benton investigate, but they are unable to turn up any evidence of paranormal activity -- until they stop into a pub after a futile day of interviews. Three hippies enter the pub after them, and when Yates catches one slipping something into a man’s drink, one of the hippies attacks him while the other two escape. Yates’ attacker is later identified as Sammy Barker, a minor criminal hired to protect the other two -- and one of the pub’s patrons, a mild-mannered bank clerk named Charles Deacon, later returns home, strangles his wife and hangs himself. The Brigadier authorises a full investigation into the incident.
The Doctor and Jo have left for a ride in the country, but the Doctor gets the impression that Jo is deliberately directing him somewhere. His suspicions are confirmed when they arrive in the village of Hob’s Haven, where pub owner Josh Bartlett informs the Doctor that there will soon be a big open-air pop concert at Rowley’s Farm. Jo admits that she came here because she missed her chance to get tickets in London, and the Doctor, amused, accompanies her to the farm to see if there are any tickets to spare. They pass a hooded figure on the way, and the Doctor learns from Bartlett that a cult called the Children of Light has set up a commune at the local abbey.
At the farm, the Doctor and Jo meet Sim Rowley, a vicious drunk who sets his dog Killer on them. The Doctor calms down the dog and proceeds to the festival site, where the harassed site manager, Tim Hollings, kindly agrees to give the Doctor and Jo all-access VIP passes for the festival. In the meantime, Rowley contacts the festival organisers and threatens to kick them off his property unless they pay him double what they originally agreed. His contacts apparently do not take well to his demands, for as the Doctor and Jo leave the festival site, Rowley’s headless body staggers out of the farmhouse and falls dead before them.
The Doctor calls the local policeman, Sergeant Bob Slater, but Slater seems remarkably unconcerned and passes the death off as an accident. When the Doctor presses the matter, Slater points out that if this is a murder, the Doctor is himself a prime suspect. Irritated, the Doctor sets off back to the village with Jo, but they are attacked by a bull in a field which seemed at first to be empty. The Doctor distracts the bull and escapes from the field with Jo, but moments later they find that the bull has vanished. At the local church, they find several recent graves, and the vicar warns them of great danger but then seems to forget what he was talking about. When a driverless tractor nearly runs them down on their way back to the pub, the Doctor loudly announces that he’s decided to leave well enough alone. The force that has been attacking him withdraws, satisfied, but its servant, the hooded figure from the village -- a familiar bearded man -- is not so sure that the Doctor has indeed been frightened off...
Back at the inn, the Doctor and Jo meet the local nobility, Lady Dempster and her grandchildren Sophie and Herbert. The Doctor and Lady Dempster both sense something odd about the other, and Lady Dempster invites the Doctor and Jo to dinner -- ostensibly as recompense for their harrowing day, but mainly to get the Doctor’s measure. As the Doctor and Jo dine with the family, the vicar arrives with an urgent message for the Doctor, but by the time the Doctor makes it to the drawing room, the vicar is dead with a dagger in his back. The family seems to enjoy guessing whodunnit, but the Doctor soon fingers the butler, Jenkins, who has scrubbed his hands raw since he met the Doctor and Jo at the door. Jenkins attacks the Doctor, but the Doctor dodges and Jenkins, pushed by some invisible force, topples over the balcony railing to his death. Slater again closes the book on the case, but the Doctor is now convinced that something evil is happening in Hob’s Haven, and that the Dempsters, though they may not be responsible, are connected to it in some way.
The Brigadier and Benton set off for Hob’s Haven to collect the Doctor and Jo while agents of MI5 and the Special Branch follow up UNIT’s lead. But Herbert Dempster, fearing that his family will suffer the reprisals, contacts their enemy to warn him that the Doctor’s friends are on their way. The Brigadier and Benton thus find their journey impeded by road signs which twist about in the wind, a river which bursts its banks and floods the road, and a sudden and inexplicable forest fire. Meanwhile, the Doctor and Jo are attacked by a fierce owl on their way back to the inn, but the Doctor drives it off with Bessie’s modified cigarette lighter, now a laser pointer bright enough to illuminate the Moon. He then rewires Jo’s UNIT radio to broadcast a homing signal that leads the Brigadier and Benton to the town. The forces impeding them make one last attempt to keep them out by generating an illusion of a stone wall around the town, but the Brigadier orders Benton to drive through the wall, convinced that it can’t be real. Reunited, the Doctor and his friends compare notes and conclude that this village is at the centre of the wave of violence. But what is the connection between the Dempsters, the pop festival, and the Children of Light?
While his exhausted friends sleep at the inn, the Doctor decides to check out the abbey, but Bartlett, fearing the power of the cultists’ leader, calls him to warn him of the danger. At the abbey, the Doctor discovers that somebody has set up a psychic fear barrier to keep people away, and also a greenhouse in which the cultists are cultivating the dangerous alien drug sarg, known to drive its victims into fits of psychotic rage. He is then captured by the cultists and brought before their leader, who, unsurprisingly, turns out to be the Master. Once he and the Doctor are alone, however, the Master reveals that he is not the true leader of the cult. He infiltrated the cult intending to take it over, only to find that their current leader, Hadley, was an powerful and dangerous immortal. The Master convinced Hadley that he would serve him loyally, and supplied him with sarg for his cultists to distribute; however, he knows that once Hadley’s plans are complete, he will have no further need of the Master. Fearing that Hadley is powerful enough to track him down wherever he attempts to flee, the Master asks the Doctor to help him escape. The Doctor instead requests that the Master remain where he is and keep an eye on the cult from the inside, in exchange for which the Doctor will try to convince UNIT to offer him amnesty for his crimes once Hadley is defeated. The Master reluctantly accepts, but is not happy with the deal.
The next morning, the Doctor tells his story to the others, who understand that the Master intends to play both sides until he knows who will win. The Doctor and the Brigadier then set off to question the Dempsters while Jo and Benton check out the festival site. The Brigadier finds Lady Dempster oddly familiar, however, and when Sophie walks in on them, the Brigadier takes one look, stands and kisses her. Lady Dempster tries to smooth over the awkward moment by suggesting that the Brigadier has mistaken them for close relations who used to live in Greece, and the mortally embarrassed Brigadier, unable to explain his actions, retreats to the car. Lady Dempster then tells the Doctor that her family has ties to the Greek mafia, who have set up shop in the abbey, but that she cannot interfere with what they’re doing for fear that they will exact reprisals on her grandchildren. The Doctor does not press her for a better explanation, but he has noted that Sophie called the Brigadier by name before they were introduced.
At the festival site, Tim Hollings informs Jo that the festival is still going ahead even though several bands have pulled out at the last moment. Benton notices a group of rough-looking people setting up in a corner of the site, and recognises many from the mug shots the police showed him after the fight at the pub. He and Jo realise that the festival is just an excuse for the cult to distribute their drug to the network of dealers in Britain. One of the cultists who escaped from the pub then recognises Benton, and the thugs turn on him and Jo. The Doctor and the Brigadier arrive just in time, and the Brigadier drives the drug dealers off with a warning shot -- but once again, Sergeant Slater overlooks the entire incident, apart from noting that the Brigadier was the one firing a gun.
Captain Yates arrives in town with news that the wave of violence is getting worse. As the UNIT folk discuss the situation, the Master arrives with new information. Hadley has returned, and the Master, knowing that he would inevitably learn about the Doctor’s visit, told him that the Doctor stumbled across their operation and wants in. The Master has also learned more about Hadley’s origins, but as he tells the Doctor about Hadley’s last attempt to take over the world, shortly after world War Two, the Brigadier unexpectedly calls out to Zeus for help and then faints. As Jo, Benton and Yates take the Brigadier back to his room to lie down, the Doctor and the Master deduce that the Brigadier must have encountered “Hadley” -- or rather, Hades -- once before.
The Doctor thus visits Lady Dempster again, and this time, insists that she tell him everything. Lady Dempster admits that she once went by the name of Demeter, and that she and her family are protected by Zeus only as long as they remain neutral. Nevertheless, she accepts that Hades has gone too far, and agrees to try to contact Zeus for help; however, she warns the Doctor that Zeus has retreated to a far-away dimension, and that he may not respond to her call for help. Herbert shares this fear, and thus secretly contacts the abbey to warn Hades about Demeter’s intentions before he finds out for himself.
The Doctor returns to the inn to find that the Brigadier has awoken -- but, still unable to recall exactly what happened to him as a young man, he is overcompensating for his moment of weakness and has called in reinforcements to arrest everyone attending the festival. The Doctor warns him that his actions will provoke a violent response from the crowd and result in the mass slaughter of innocents. At best, his career will end in disgrace; at worst, the incident will serve as the flashpoint Hades needs to spread his influence over the entire world. However, the Brigadier insists upon doing his duty as he sees it.
Frustrated, the Doctor returns to the Dempster mansion, only to find that Lady Dempster has failed to contact Zeus -- and that Hades has kidnapped Sophie to stop her from trying again. Ashamed of his cowardice, Herbert agrees to help the Doctor rescue his sister. The Doctor goes in first to keep Hades distracted, and discovers that the sarg has been harvested and processed; soon it will be on its way to the pop festival, from where the drug dealers will distribute it across Britain. The Master, fearing the consequences should the Doctor fail to defeat Hades, gives the Doctor a chance to join their side for real, but the Doctor rejects the offer; nevertheless, he does manage to convince Hades that this is just what he intends to do. Satisfied, Hades dismisses the Doctor, unaware that, while the Doctor was distracting him, Herbert entered the abbey and located Sophie. Hades has placed her in a magical coma, and as the Doctor and Herbert try to carry her to safety, the Master catches them escaping... and, rather than turning them over to Hades, he escorts them to the abbey’s outer door and allows them to slip away quietly.
Back at the Dempster manor, the Doctor outlines a plan to boost Lady Dempster’s telepathic power: he, Sophie and Herbert will join their minds together, pushing Lady Dempster’s consciousness out as far as she can go and allowing her to launch from there. Thus, she will be able to reach out further than she could have alone. Sadly, when the Doctor wakes from his telepathic trance, he finds that Lady Dempster has fallen into a coma. He must set off for the pop festival by himself, leaving Sophie and Herbert to tend to the comatose Lady Dempster.
UNIT troops cordon off the festival site, but the Brigadier is beginning to realise that there are more people here than he can handle peacefully. Jo tries to slip into the grounds to see some of the festival before midnight, but the drug dealers are out in force, handing out free samples of sarg, and Jo soon finds herself trapped as the audience turns into a raging mob around her. The Doctor arrives just in time to rescue her, but the Brigadier remains determined to impose order on the crowd, despite the Doctor’s warning that things have gone too far and that the Brigadier is about to go down in history as a butcher of civilians. He does convince the Brigadier to wait until the stroke of midnight, at which time Hades manifests on stage and orders his followers to rise up against their oppressors. Swayed by the drugs and violence, the crowd turns on the UNIT troops, tearing apart Private Higgins with their bare hands and advancing on the others. Just as an all-out battle is about to break out, however, a great voice speaks from the sky, banishing Hades back to the realms of darkness forever. A monsoon rain sweeps across the festival, washing away the mob and breaking the mood of violence. At the Doctor’s request, the Brigadier sends in his troops, but to provide a relief effort rather than to open fire.
The next morning, the Doctor and the Brigadier visit the Dempster manor to find Lady Dempster hale and hearty; as the Doctor suspected, she did in fact manage to contact Zeus and call him back to save the day. She and her family have now decided to join Zeus and leave Earth behind, but before leaving, Sophie bids a fond farewell to Alistair, who knows he will never again forget her first love. As Yates and Benton oversee the relief effort and arrange to place the drug dealers under arrest, the Doctor, Jo and the Brigadier leave the village, only to spot the Master slipping away. The Brigadier concedes that the Master saved the day by allowing the Doctor to escape from the abbey with Sophie, thus enabling Lady Dempster to contact Zeus, and thus, albeit reluctantly, he allows the Master to depart unmolested in the name of their temporary truce.