Alice Wendell is in the great glass dome of the Oxygen Gardens of Chelsea 426. The gardens are run by Professor Wilberforce who came to Saturn with the Interplanetary Mining Corporation when Chelsea 426 was first started as Unit 426, and he has stayed on since the takeover by Powe-Luna Developments who have refurbished the settlement into the shape of a twentieth century English market town.

The gardens are busy on this day, 20 August, as the botanists prepare for the influx of visitors to the Flower Show. Alice finds the professor looking at his largest specimen, 'Blue Flower of Saturn', a plant that stands nearly four metres in height. Alice approaches him and reports that she has detected unusually high ammonia readings in the atmosphere of the dome. He leads her to his office and extols the wonders of the spores he discovered in the atmosphere of Saturn. He wonders how long they have waited to be discovered and where they came from originally. He lifts the cover from one of the plants in his office and it spits out a green cloud of spores which Alice breathes in. She collapses but revives later, and seems transformed. She and Wilberforce discuss the surprising presence of humans in this sector but say that the arrival of thousands more will lead to the growth of their own numbers.

The Grand Hotel had been hoping for an influx of guests come to visit the Flower Show but it has remained empty since most of the tourists are staying on the luxury space liners that have brought them to Saturn. The Carstairs family owns the hotel and their children, Jake and Vienna, spend much of their time keeping the place spotless for visitors who never arrive. Only one of the 150 rooms is occupied. It is something of a surprise when the Doctor walks in and asks if there are any vacancies. No sooner has he booked himself in than he asks if he can accompany Jake and Vienna on a trip to the hardware store to buy polish.

He discovers that the children have been on Chelsea 426 for two years but find the bizarre recreation of twentieth century England more than a little boring. The children repeat the Colony Code to the Doctor, six rules that seem to add up to a restriction on fun of any kind. In the hardware shop, the children are greeted cheerfully by the owner, Mr. Pemberton, but he regards the Doctor with mistrust and after they leave he watches them pass down the street.

Jake begins to tease Vienna because she has a crush on Wallace, the boy who works in the hardware store, but is interrupted by the Doctor who sees a familiar face on a public television screen. Jake tells him that it is Riley Smalls, a television commentator and self-appointed guardian of public morality. The Doctor is surprised; Smalls was doing much the same thing five hundred years earlier on Earth. Vienna merely informs him that Smalls is a Cryogen, and acts surprised that the Doctor doesn't know this. Changing the subject, the Doctor asks where all the flowers for the show are from, and Jake tells him about Professor Wilberforce discovering the spores. The Doctor decides to call in on the professor.

Riley Smalls has been called into the Mayor's office where the mayor, Mr. Sedgefield, is concerned that the broadcaster is being unnecessarily critical of the influx of Newcomers for the Show. Smalls says he is just reflecting a public worry that many of the Newcomers will stay on after the end of the Flower Show. He implies that if Sedgefield does not ensure an end to visitors after the Show then Sedgefield won't be mayor for much longer.

Posing as a member of the Intergalactic Horticultural Society, the Doctor talks briefly with Alice Wendell who tells him that a sneak preview ahead of the official opening will not be agreeable. She then reports the meeting to Wilberforce who admits that his telepathic link was not strong enough to listen in to the conversation. The professor makes a call to Pemberton, who reports his meeting with the Doctor, a name that interests the professor.

Later, in the bar of the Grand Hotel, the other guest, an old Englishman called The Major, is regaling the Doctor with tales of the Battle of Olympus Mons. The Major confides that he isn't a major at all and the Doctor laughs before saying that is a coincidence.

Meanwhile, Pemberton takes Wallace to the Oxygen Gardens on the pretext of giving him a preview of the Flower Show, only for Wilberforce to repeat his trick of releasing the spores to possess the boy. The following morning sees Wallace arrive at the Grand Hotel to present Vienna with four free tickets to the Flower Show. Her father says he can't go but suggests the twins and their mother make use of the tickets. Much to her mother's displeasure, Vienna promptly offers the spare ticket to the Doctor.

The Doctor spends his first few minutes in the Oxygen Gardens looking worried and tells Jake that if he shouts "Now" the boy must get his mother and sister out as quickly as possible. Soon after this the Professor unveils his gigantic blue flower which promptly begins to writhe and emits a green cloud. Many of the crowd which has gathered in the gardens collapses choking. This group includes Mrs. Carstairs but not the twins who the Doctor hustles away. Instead of leaving with the panicked majority the Doctor stops at the exit and doubles back, returning to the twins with their unconscious mother.

He bursts into the Grand Hotel and lowers her into a lobby armchair next to the Major. He tells Mr. Carstairs that she needs to be taken up to his room. As the Doctor and the four Carstairs ride off in the elevator the Major takes over at reception, muttering about the Siege of the Hexion Gates.

The TARDIS is parked in the corner of the Doctor's room, to the consternation of Carstairs - mainly because it is bigger than the doorway. The Doctor enters the TARDIS and rummages about. As he does so the twins peer in through the door and laugh with disbelief. The Doctor emerges with a small device and examines Mrs. Carstairs, detecting the spores. As he wonders where he has seen them before the woman regains consciousness. Her husband offers her a cup of tea to which she responds "That would be agreeable". The Doctor recognises this odd phrases something he heard Pemberton and Wendell saying. He says it is like human speech but not quite and declares that Mrs. Carstairs has been taken over by the spores. This cuts no ice with the Carstairs who leave, huffily. However, Vienna says she heard Wallace use the same phrase when he gave her the tickets. Their conversation is interrupted by the arrival of a Sontaran ship above the colony.

Mayor Sedgefield orders all five of Chelsea 426's policemen to arm themselves but not to open fire on the soldiers who are appearing in the colony. However, when the police face ten of the Sontarans they are not given a chance. Colonel Sarg of the Fourth Sontaran Intelligence Division states his intention to search the facility for enemies for enemies of Sontar. He notes the police's weapons and orders his men to fire. The Doctor arrives too late, telling the police to drop their guns but before they can respond they are massacred in full view of the crowd of locals and Newcomers that have gathered in the square. The Sontarans then turn on the Doctor.

General Kade is shown into the Mayor's office. The Sontaran announces that intelligence shows the colony to have been taken over Rutans. The Sontarans have sealed off the colony and locked all the docking systems to prevent any ships from leaving. He adds that the colony will be left in peace once the Rutans are all dead. Sarg enters, holding the Doctor at gunpoint. He says that the Doctor has demanded to speak to the General. Kade is pleased to see the Doctor and introduces him to Sarg as a remarkable face-changer with the knack of outwitting the Sontarans.

Kade explains the back-story to recent events. The Sontarans had planned to use the Earth as a cloning facility to breed a new army in this sector of the galaxy. The Rutans had discovered this plan and planted spores in the solar system that would have allowed them to infiltrate the clone army when it eventually transpired. Plans changed, however, and Earth was never taken by the Sontarans so the humans were the ones to be infected by the spores.

The General speculates that it would be easier to just destroy the whole colony and kill everyone. The Doctor tells him he will not allow that but Kade points out that the Doctor is leaving: the Sontarans are permitting him to make his exit before they take any further action. The Mayor interrupts to add that any Rutans are likely to be among the Newcomers who were at the Flower Show.

While the Newcomers are being rounded up the Doctor returns to the Grand hotel to confront Mrs. Carstairs. He speaks to her as though she was a Rutan and demands that they stop using the humans as part of their wart effort. The twins enter and report that something interesting is on television. Riley Smalls is holding forth saying that the Sontarans are actually the saviours of Chelsea 426 and should be welcomed for removing the Newcomers. The Doctor notes that early Cryogens are noted for their bad temper and poor judgment: most of them are kept in nursing homes on Earth.

Aboard the Pride of Deimos, the pride of Earth's luxury liner fleet, a young couple - Zack and Jenny - who could barely afford the tickets for their honeymoon, are gathered on the deck with the rest of the passengers when the Sontarans arrive to disembark them for investigation. They are taken to the loading bay of the colony wher4e Professor Wilberforce is testing them for spores, under the eye of the Sontarans. When Alice Wendell, who is helping the professor, lets slip an 'agreeable' General Kade becomes suspicious. The professor promptly kills one of the Sontaran soldiers before he and Alice are killed by Sontaran laser fire. Kade orders the incarceration of all humans on the colony.

In his dressing room Riley smalls considers how the Newcomers, terrifying as they are to him, have relieved the boredom of life in the colony. When he starts his show it is with a diatribe against the Newcomers but this is interrupted by the arrival of the Sontarans to take away the humans. Smalls protests that he has supported the Sontaran invasion but they take him anyway.

Watching in the Hotel, Mr. Carstairs wonders what they can do to evade the Sontarans. The Doctor thinks, briefly, of using the TARDIS to get away but discounts the idea. The Major, on the other hand, suggests that he get his gun. Mr. Carstairs tells him to be quiet and that he is fed up of the Major's interminable war stories before asking the Doctor to use the TARDIS to get the twins away from the colony. Reluctantly, the Doctor agrees. He takes the twins and the Major up to his room. As they step into the elevator the Sontarans arrive and take the Carstairs adults prisoner.

In the corridors of the upper floor the Major tells the Doctor that he does not approve of running away from the enemy. He remarks that this reminds him of the Siege of the Hexion Gates, much to the Doctor's lack of interest. At this point a Sontaran soldier steps into the corridor and orders them to put their hands up. The Doctor tells the twins to comply as does the Major, grumbling that he would never have surrendered in his day. The Doctor reminds him, impatiently, that they are not in his day any longer. However, when the Sontaran turns to address the Doctor it gives the Major the chance to hit its probic vent with a rolled up newspaper. The Sontaran collapses and the Doctor asks the Major how he knew what to do. The Major replies that the Siege of the Hexion Gates was a battle against the Sontarans.

Sarg reports to General Kade that all but a few of the humans are in custody in the docking areas and the plants are about to be destroyed. He argues that it would be simpler to kill everybody but Kade reminds him that they are intelligence officers and need to gather information. The Doctor's arrival surprises them and he asks for a private word with the general. He again asks that the humans be lefty alone but Kade says none of the humans shall be killed unless they are hosts to the Rutan spores. These will be tortured until they give up their secrets. The Doctor leaves angrily, uttering a final warning as he goes.

Hiding in a store cupboard, Pemberton and Wallace decide to use the ducts to gain access to the TARDIS and leave the colony. Wallace makes it to the Hotel and enters the Doctor's room. He rings Vienna on her chatcom device and asks to be let into the TARDIS. Vienna persuades her brother that Wallace is not a Rutan but as soon as he enters his hands produce an electric charge which he aims at Jake. Before he can let it go the Major wrestles him to the floor. They become a crackling ball of energy from which Wallace emerges, leaving the Major still on the ground. At this point the Doctor enters the TARDIS. Wallace turns to attack him but suddenly lurches and falls. Vienna screams that Wallace is dying but the Doctor explains that the TARDIS air-conditioning has removed the ammonia from the air and the Rutan spores are dying.

The Doctor turns his attention to the Major and thanks him for saving the lives of the twins. The Major mutters his farewells, saying that he is glad it wasn't a Sontaran that killed him. Then he adds that Sontarans have small ears and it only takes a little sound to deafen them. He tells the Doctor to work out what that means before he passes away. Wallace recovers consciousness, bemused about recent events and unaware of what Sontarans actually are.

Mrs. Carstairs detaches herself from her husband in the loading bay and walks over to a group of other colony residents. Her husband strikes up a conversation with Zack and Jenny despite his dislike of Newcomers. He wonders how his wife is so familiar with the other residents, especially as he doesn't even know them by name. He finds himself reassuring the young newly-weds in front of him. A group of Sontarans come to take the couple for questioning but they are distracted by Riley Smalls who says he knows where some Rutans are hiding. He leads a party of soldiers to his apartment in a luxury pod above the Western Docks. Once he has them safely locked in his apartment Smalls opens the emergency exit which vents the Sontarans and then Smalls into the icy blackness of space.

Back in the TARDIs the Doctor ponders the Major's final words, and then explains to the children that his TARDIS can make a high pitched noise that only deafens Sontarans. All he needs is somewhere that can play the noise to the whole colony. He hits on an idea and leads them into the ducts to the television studios.

The Sontarans destroy the Rutan plants. Then, news of the death of six soldiers reaches Sarg he is furious at the treachery of the humans and declares Kade a coward for not killing everyone. He goes straight to the General, demanding action in reprisal. Kade accuses him of mutiny for which there is only one course of action. The two Sontarans and their higher ranking officers make their way to the metal ramps around Chelsea 426's fusion candle. Sarg and Kade fight in single combat until Sarg plunges to his death from a bridge above the candle.

The humans in the loading bay are oblivious but their Sontaran guards are suddenly deafened by an almighty noise. As they roll in agony the Rutan humans leave the bay. Carstairs watches his wife go with them.

In the fusion chamber only Kade has the strength to resist the noise. He takes a rifle and hoots out the speakers one by one. The general knows who made the noise and where he will be. He leads his men through the colony. The Doctor watches this on monitors and declares it is time to leave the studios and get rid of the ammonia from the air. He takes his young friends to the Oxygen Gardens.

On the streets the Rutans and the Sontarans are fighting face to face. The Sontaran guns are useless against the Rutan energy waves. Carstairs is the sole witness, despairing that his wife is now an alien. When Kade hears of this impotence on his men's part he knows there is only one way to end things. Carstairs follows the Rutans back to the gardens where they survey the ruined plants.

Inside the gardens the Doctor cannot access the Rutan computer programs until Vienna steps in to help. With the Climate Control at his command the Doctor filters the ammonia out of the atmosphere. The twins see their parents and some other residents enter the gardens and run down to meet them, only for Mrs. Carstairs to take Vienna by the throat and threaten to kill her if the Doctor does not hand over the TARDIS. The Doctor smiles and notes the freshness of the air as the host bodies collapse and then recover with no memory of recent events. Any celebration is halted by the arrival of Kade and his men. They have decided that the Doctor is on the Rutan side and prepare to fire until Mr. Carstairs steps between them and feigns the Rutan defensive posture. Thinking that his men's weapons will be useless Kade tells the Doctor that he has realigned the colony's flotation panels and soon it will be drawn into the Great White Spot, a storm on the surface of Saturn that is larger than the Earth itself. Chelsea 426 is doomed. At that, the Sontarans beam back to their ships.

Rushing to the Mayor's side the Doctor is horrified to hear that there are not enough life rafts for the residents nor is there space aboard the liners to take them. Jake has an idea to use the rockets of the moored liners to steer the colony back to safety. The captains of the ships agree to this and after a tense and lengthy series of maneuvers the job is done and the colony is saved.

Because of the damage incurred by the rescue attempt, and the resulting lack of fuel, the ships are forced to stay at the colony for several days. This is good news for the Grand Hotel which is full of guests, including Zack and Jenny who are given a room for nothing by Mr. Carstairs. More good news comes for Vienna when Wallace asks her to go out with him. The Doctor takes his leave of the twins and their parents who inform him that with the money from this unexpected windfall they intend to relocate back to Earth.

Entering the TARDIS the Doctor sees a news headline about the invasion of the colony that includes the news that the Major was, in fact, Field Marshal Sir Henry Whittington-Smythe, a war hero on several counts, and due a full military funeral. After a quick look around the empty console room he pulls a lever and dematerializes the TARDIs.