June is a classics student who has decided to visit Greece for a holiday so that she can visualize something of the ancient world. On the last morning of her visit she stops off on the way to the station to see the Acropolis. For a while she is alone at the site, and then some other tourists begin to arrive. As she is looking into the theatre below the Parthenon she sees the TARDIS materialise and the Doctor hurrying to some caves nearby. Curious, she follows. In the caves she sees the Doctor speaking to a group of blobby grey aliens. He is telling them to leave the planet, saying that they have enough explosives with them to destroy the Parthenon. Determined to rescue the Doctor from the aliens, June races at them, roaring. They immediately run into a silver sphere and the Doctor locks them in with his sonic screwdriver.
June asks why aliens are trying to destroy the Acropolis but the Doctor is busy disarming the explosives. He adds that the aliens said they had a good reason for what they were doing but didn’t have chance to say what. The couple introduce themselves properly and explain their backgrounds. June tells the Doctor about wanting to get a feel for Greece for her studies and the Doctor decides to take her back in time.
He sets the controls of the TARDIS for 480BC but on the journey he spots a distress signal and an anomaly around 1500BC and decides to investigate. As the TARDIS hits the anomaly there is a lot of bumping and an explosion. After June recovers she sees steam escaping from the console and the Doctor paralysed beside it. All he can do is say, ‘Out.’
She helps him out of the TARDIS into the cold night air. She recognises the shape of a nearby hill as being the site of the Parthenon in the future and then the TARDIS door clicks shut behind them. Before she can look for the key they are attacked by three lions. The Doctor revives briefly to throw a lioness to the floor and it runs away, followed by the other two. He then reverts to his uncommunicative state. A chariot arrives, driven by two warrior girls who load June and the Doctor onto the back and take them to a walled settlement on the future Acropolis. The warriors take the Doctor into a hut lit by a fire in the centre. An old man within immediately surmises that June has arrived from the future and introduces himself as King Acteus. He tells her that his province has fallen on hard times since an earthquake. The warrior girls return with food and he introduces them as Aglauros and Pandrosos. After the meal Acteus shows June some paintings that depict the history of his people. In one painting are some aliens next to a spaceship. He explains that they are Slitheens and are the masters of the humans. At this the Doctor revives.
June is delighted by the Doctor’s return to life and introduces the king and the two princesses. The Doctor is perplexed that Acteus readily accepts that they are from the future and asks what has been happening. Acteus tells him that the Slitheen have been on Earth for more than a generation. They have been providing food for the locals in return for the provision of young warriors who are never heard from again. Acteus has already lost two sons (one of whom was married to Aglauros). After their departure an earthquake ruined the land. The masters visit occasionally, usually with a variety of other creatures. June realises that the Slitheen are running a package tour of ancient sites for aliens. Acteus reveals that the Slitheen have now demanded his daughters in payment.
The Doctor refuses June’s plea to help the king, saying that the Slitheen may always have been a part of the history of the region. He announces his intention to return to the TARDIS. During the night the Doctor and June sneak back to the TARDIS (taming a lion into submission by tickling it with sonic waves from the screwdriver en route). I the TARDIS the Doctor theorises that the many monsters of Greek mythology may have been the aliens that are now on Earth with the Slitheen. He looks at a screen showing spatial bending – probably evidence of transmat activity in the region – but no large towns. He thinks that human progress is being held up by the Slitheen and it is this that decides him on staying to investigate further. They return to the walled enclosure and the Doctor offers himself and June in place of the king’s daughters.
They take a chariot to the nearest beach where they find a party of other warriors already assembled. Some of the warriors try to start a fight but the Doctor persuades them that this is what the Slitheen want and says that they all have a better chance if they work together. There is soon a lot of bonding over a barbecue and some wine. The next morning eight boats arrive, unmanned, in the harbor and the warriors board the, ten on each. The boats sail smoothly out to sea. Unfortunately a huge storm blows up and the boats are all capsized. Few of the warriors can swim and so most of them drown. June rescues one, Deukalion, and the Doctor retrieves another, Vik. After the storm abates they find that two of the boats are still afloat and that a man, Polos, and a woman, Herse, have also survived. The boats resume their journey with the six survivors aboard.
They arrive on a rocky coastline. On the cliff above them is a tall stone building but there is no way up from the beach. They wait on the sand as the tide comes in, washing around them. They realise that the incoming tide would have panicked a larger group and started another fight for survival. They think it is a trick of the Slitheen to wed out weaker warriors. However, the six of them work together to stay afloat until the cliff opens above them. They climb into a large cave. Inside is a pool of water containing a copy of the Cutty Sark. Beyond is a long passage that they follow. It takes them up to a large courtyard full of other warriors. These warriors are wearing clothes from across Earth’s history. On a gantry above stand a crowd of alien tourists laying bets on how well the warriors will perform. The Doctor, June and Deukalion find a doorway and use the sonic screwdriver to gain entry to the cloisters beneath the gantry but are apprehended by one of the Slitheen.
The Slitheen grips the Doctor by the throat but June says that this sort of behaviour will lead to a bad review. Intrigued, the Slitheen releases the Doctor who explains that he is travelling incognito as a journalist for the Mutter’s Spiral Herald. The Slitheen, Cosmo, falls for this at once and offers them a guided tour of the Slitheen operation to help the Doctor write his article. They are shown round a 1300 room complex full of historical artifacts. They are then led up to the gantry to meet the alien holiday-makers. Cosmo tells the Doctor that he is from the year 34,600, just after the Platonic War, but the Slitheen are still in exile from their home planet. Deukalion is upset to hear that the ‘masters’ have no control over the earthquakes which Cosmo claims are a nuisance that have nothing to do with the Slitheen operation. He says that the humans are not being kept back deliberately from progressing as they should; in fact the Slitheen are using molecular repurposing to create food for the starving masses that they found on their arrival. The humans had apparently forgotten how to farm their own land and have shown no inclination to learn. Cosmo claims that if it were not for the revenue from tourism the whole of the human race around the Mediterranean would have already starved to death.
Returning to the tour of the facilities they encounter Cosmo’s three siblings. Unfortunately, these three are not taken in by the Doctor’s story of being a journalist. Mamps, Cosmo’s elder sister, decides that the time has come to revive the gladiatorial games with the Doctor as the first competitor. The Doctor is taken into the courtyard and introduced to the alien spectators by another Slitheen, Hisk. A bull enters the courtyard and charges at the Doctor who evades its charge by vaulting over its back. He repeats this feat a number of times until the bull is exhausted. Hisk is furious, claiming that the viewers must have a death to watch. His tirade is cut short when the bull charges him, killing him instantly.
Cosmo is very upset with the Doctor but he has to acknowledge that the Doctor’s exploits in the arena have made him popular with the alien tourists so it won’t be possible to simply kill him. Instead, the Doctor has been invited to a banquet with June and Deukalion. The meat for the banquet is from the bull that killed Hisk. June shudders to realise that if the bull had killed the Doctor the aliens would have been eating her friend. Her thoughts are interrupted by a fish-tailed humanoid called Cecrops who seems to have taken an interest in her. He seems to be upset about the way humans are being exploited for the entertainment of the tourists. Mamps interrupts to say that all the humans are volunteers and that to ensure there is no exploitation the Doctor is going to be the humans’ ambassador. He is going to be taken over to the island where the Slitheen transportation equipment is located. June warns him that it is a trap and they just want to get him away from everyone else so that they can kill him. He tells her that he knows this already.
Mamps and Cosmo use the transmat to take the Doctor to their temporal drives. The Doctor notes that it is made up from the technology of many different races. He also realises that the Slitheen don’t actually know how it all works. He says he knows that they intend to kill him, which is a shame because he could actually improve their machines and stop the earthquakes. He tells them that these are the product of their use of the temporal drive. Mamps confesses that they used Navarino technicians to build the drive but then they ate them. The Doctor says that if the drives break down the Slitheen will be trapped but Mamps explains that her family in the future has a time bus for emergencies.
June is rather disappointed that Cecrops interest in her is not personal but because she symbolizes his cause. She wanders away from his attentions to talk to Deukalion and they are approached by the remaining Slitheen, Leeb. He tells them that are to volunteer for a show he is putting on. If they refuse he will kill Acteus and his daughters in a huge explosion. June says that if the site of the future Athens is destroyed then human history will be changed in ways that can’t be imagined. Leeb disagrees, saying that he knows exactly what would happen: humanity would never become a space-faring race.
The Doctor makes some changes to the programming of the temporal drive, confident that the watching Mamps has no idea what he is doing. Part of his repair is to eradicate the earthquakes but he has something else up his sleeve. No sooner has he finished than Mamps pounces to kill him.
In the arena twenty five humans are dressed as aliens and have to fight another twenty five dressed as human warriors. The show is a reconstruction of the Platonic Wars where the human forces carried out a number of atrocities, making them a despised race. In this reconstruction, however, June has decided to forego senseless slaughter and makes the human army run away and surrender with no bloodshed. Far from being disappointed by the lack of bloodshed, the watching alien tourists are delighted by the outcome. When Herse invites the spectators down into the arena to join them for canapés and cocktails there is a delighted reaction. Leeb is furious that he did not get the wholesale massacre he as hoping for. Worse, he has to produce refreshments out of nowhere and the humans are talking to the aliens about the conditions that they are being kept in. He calls Mamps for advice. She is just about to kill the Doctor when she takes the call. He says that he can help her again because he has experience of negotiating with humans. Reluctantly, Mamps agrees to let him live for a little longer.
Deukalion uses an unguarded transmat to find his way to Acteus. He returns with the king and his two daughters. They arrive in the Slitheen store rooms just before the Slitheen return with the Doctor. The Slitheen head straight for the arena. There the Doctor secretively praises June for her cleverness in getting the humans and aliens talking. The Slitheen are working up a plan where they can accuse the liberal Cecrops of corporate espionage when the sky explodes.
Acteus and his daughters have made their way to the temporal drives, transmatting vast quantities of oil with them. Acteus has started a fire, sacrificing himself and causing the entire chamber to explode.
The Doctor and June watch the huge explosion, realising that the entire island of Thera, where the drives were, has gone. Thousands of people will have died. June recalls that modern Thera is now known as Santorini. Mamps orders Cosmo to send a distress signal. The Doctor follows him and reroutes the power from the signal to create a force field just in time to ward off a massive tidal wave that threatens to swamp the Slitheen holiday home. The aliens are very grateful to the Doctor and Cosmo for saving them but turn on the Slitheen for being the reason why Acteus would take such desperate measures. Their sympathy with the humans is compromised when a party of warriors decides that talking to the aliens will not bring them what they want and launch an attack instead. A massive brawl breaks out: aliens and humans are fighting while other aliens and humans try to intervene and find a peaceful situation. The climax is manufactured by Cosmo who sneaks away to the sleeping quarters and returns with an alien baby. He presents it to Aglauros. She threatens to kill the baby in return for the child she lost to the Slitheen. This brings the fighting to a halt. June manages to persuade her to hand the baby back to its mother. This convinces the aliens that the humans are civilized. Before anyone can turn on them the Slitheen transport themselves away.
June expects the Slitheen clan to arrive but the Doctor tells her that he redirected the distress signal back to himself. June realises that this was the signal they saw as they stepped into the TARDIS in modern Athens. She also works out that the time anomaly that debilitated the Doctor was the temporal drives exploding.
In order to get all the aliens home the Doctor loads everyone, alien and human, into the Cutty Sark. He then sails to the remains of Thera. On the way they pick up a hundred survivors of the tidal wave. Together, everyone dismantles the ship and takes the pieces up to the highest point on the island. They lash together a mile long platform. At the end is a rip in time. The aliens make their way along it and disappear to their future homes. Only Cecrops remains behind saying he has nothing to return to. He knows that if the aliens say he died on Earth his family will take the Slitheen family to court and bankrupt them.
Just as the last aliens leave the Slitheen arrive. They threaten to kill all of the humans but the Doctor warns them that the temporal fissure is about to close. Mamps rips June’s stomach with her claws and then the three Slitheen run along the platform and dive at the rip in time. It closes before they get there and they plunge to their apparent deaths. June, too, feels her life ebbing away.
She awakens on the rock that will become the Acropolis. Her wounds have been slowly healing and the Doctor has been using the time to teach the Greeks to farm for themselves. After one final party the Doctor and June make their way to the TARDIS and head forward in time to Athens as it nears its height of power. June is delighted to find the vibrant city as she had hoped. They visit the Parthenon. A tour guide tells them the history of the city: it was founded by King Cecrops, half man half serpent, and his three daughters (Aglauros, Pandrosos and Herse). June is saddened to learn that the daughters were turned to stone by the Medusa but when he shows them the statues of the daughters June recognises them as Mamps, Leeb and Cosmo.
As they return to Athens at the moment they first left a message arrives. The Doctor finds that the aliens he stopped from blowing up the Acropolis were the space police. He races back to the cave where the alien ship and the explosives are still in place. He reactivates the fuse and the ‘bomb’ melts three stalagmites which collapse to reveal the three Slitheen. Far from being dead for thousands of years the Slitheen pounce.
After a frantic chase round the Parthenon the Slitheen are apprehended by the space police. June is taken for questioning by the Greek authorities but when she can supply no answers they let her go. She makes her way back to the TARDIS to find that the Doctor has been helping the blobby aliens. It turns out that Cecrops’s plan worked and the Slitheen family is out of business.
June realises that she has missed her train home to Birmingham but the Doctor offers her a lift in the TARDIS.