After trying to warn young American genius Luke Rattigan that his car navigation product ATMOS is dangerous, a journalist named Jo Nakashima is thrown from the academy that bears the boy’s name. She clambers into her car and prepares to leave for UNIT HQ, guided by the ATMOS device installed in her car.

As she drives away Luke returns inside the ‘Rattigan Academy’ and speaks to an unseen figure via an intercom. He warns that the journalist could pose a threat to their plans and an agreement is made to terminate her. Sure enough, some distance away, the ATMOS device in Jo’s car takes control of the vehicle, and plunges her into a river…

Meanwhile, in the TARDIS, the Doctor looks on nervously whilst Donna attempts to pilot the ship but is distracted by the ringing of a mobile phone. He realises it is the one given to him by Martha, and answers the call.

“Doctor? It’s Martha. And I’m bringing you back to Earth.”

The TARDIS arrives in an alleyway somewhere in London, where Martha is waiting. She greets the Doctor warmly and then introduces herself to Donna. The two women speak as friends, which comes as a great relief to the Doctor, who assumed they wouldn’t like each other. Donna notices an engagement ring on Martha’s finger and she reveals she is to be married to Tom Milligan, whom she met during the Toclafane Invasion.

Once the welcomes are over Martha produces a radio from her jacket and calls for “Operation Blue Sky” to commence. Seconds later the area is swarming with UNIT troops, who storm into a nearby factory and order the workers to surrender. Martha strolls off to oversee the procedure, leaving the Doctor somewhat bemused that his former companion has become a soldier.

However, the Doctor and Donna are not the only ones watching the unfolding scene. The disembodied voice that spoke to Luke is also observing, and talks with a fellow conspirator. They claim all is going to plan – the UNIT threat will not impede them.

Now a fully trained doctor, Martha has become an expert on alien activity and is working for UNIT – a name that now stands for Unified Intelligence Taskforce. She takes the Doctor and Donna inside a UNIT lorry, which houses a mobile HQ. The Doctor is introduced to Commanding Officer Colonel Mace, who claims that because he hasn’t resigned from the post, the Doctor is still UNIT’s scientific advisor.

The Doctor asks what UNIT are doing at the factory, which is owned by a company called ATMOS. Colonel Mace explains that the previous day fifty-two people died simultaneously when their cars drove into open water. There are no wounds on the bodies and any poisonous toxins that may have been present were dispersed immediately so the only thing linking the victims was the fact all their vehicles were fitted with ATMOS – which stands for Atmospheric Emissions System.

Donna, who has heard of the system before, assists Martha in explaining that the new technology makes exhaust fumes in vehicles 0% toxic. Such an incredible feet of engineering has aroused suspicion, and the murders have forced UNIT to investigate the possibility that ATMOS may be alien.

As UNIT troops begin rounding up the workers and investigating the factory, the Doctor is shown a model of the navigation system, which has tested negative for any alien threat. Donna questions why aliens would wish to produce environmentally friendly technology, and the Doctor explains that the devices may have been built as concealed weapons. If one is attached to each car on Earth, than whoever is running this scheme will have an 800 million-strong arsenal at their disposal.

In the lower levels of the factory two soldiers investigate a darkened corridor. After finding a light switch they make their way to a room guarded by two of the ATMOS staff. They order the men to stand down, but it is only when the disembodied voices that spoke to Luke order the men to do so, that they step aside.

Entering the chamber the two soldiers discover a vat of bubbling green liquid, from which springs a half-formed humanoid figure. They back off and examine it from a distance. Seeing an umbilical cord running down it’s back they begin to assume it is being grown, and that the green liquid is it’s food. Suddenly a somewhat squat alien appears behind them. Dressed in full body armour and a domed helmet the creature congratulates them on their intellect then fires an alien gun at one of them, causing him to keel over in agony.

The other soldier tries to open fire with his own gun but the creature warns him that a special electromagnetic field covering the area is causing the metal of his bullets to expand inside the barrel, thus rendering the weapon useless. Similarly, the soldier’s radio signals have been blocked. He shoots the other intruder and motions for the two ATMOS guards to enter, claiming the UNIT troops are to be used as part of his stratagem. Upon being asked his name he claims he is General Staal of the Tenth Sontaran Battle Fleet. He then removes his helmet, revealing a slimy brown face underneath.

Back in the UNIT lorry the Doctor continues examining one of the ATMOS devices under the watchful eyes of Martha. After they air their views on the military situation (the Doctor admitting to being uncomfortable with so many guns around him), Donna enters and claims she has discovered something interesting. She explains that she has been investigating one of the ATMOS offices, and has uncovered a file detailing the absences of staff members – it is empty – not one worker has reported ill since the company was founded, and that has aroused her suspicion. Martha complements Donna on her investigation then, acting on orders given by Colonel Mace, goes to examine one of the workers. Once she and Donna have gone Mace talks to the Doctor about Luke Rattigan, a renowned child genius who invented ATMOS. He now runs the Rattigan Academy, a school for exceptionally gifted children.

Now alone and able to talk, Martha warns Donna that she must warn her family of the travels she is embarking on with the Doctor – because the consequence of life with him can be devastating. She explains that her own family endured torture once she had left them behind and, although not the Doctor’s fault, they need to be protected. Donna agrees to return home, already concerned that her mother has an ATMOS device installed in her car.

Back in the guarded chamber General Staal orders the now-brainwashed UNIT soldiers to take their positions in time for the completion of the Sontaran stratagem. He watches them go then steps into a large square purple pod. He claims that soon the planet Earth shall witness its last days, then disappears as the pod, a teleport, activates.

In the main facility the Doctor decides to visit the Rattigan Academy to talk with Luke, and Colonel Mace grants him access to a jeep, along with an escort, Private Ross Jenkins. Donna arrives and turns down the opportunity to go with them; instead she has decided to return home. After lamenting the places they could have travelled to together and thanking her for saving his life in more ways than one, the Doctor realises his companion only intends to return home for a brief visit, not forever. She mocks his misinterpretation then clambers inside the jeep, claiming he and Ross can give her a lift home.

As they drive away Martha prepares to inspect one of the factor workers. When she examines his heart she discovers it is beating at an alarming rate. She interrogates him and, speaking in a cold dull monotone, he reveals that he works twenty-four hours a day. Her suspicion aroused, Martha decides to talk with Colonel Mace. Two UNIT soldiers intercept her to tell her Mace is waiting for her elsewhere in the factory. She lets them act as escorts, unaware they are the two guards brainwashed by General Staal.

Having been dropped off by the Doctor and Ross, Donna walks along the street in which she lives and recalls in her mind the adventures she has faced over the past few days. She eventually arrives at her home, where she is welcomed with open arms by her grandfather Wilf.

Back at the factory the conditioned UNIT guards escort Martha to the hidden chamber in which is contained the teleport and the vat of green liquid. She steps inside and, upon failing to see Colonel Mace, realises she has been led into a trap. She tries to leave but the guards prevent her. They drag her back inside and the door closes behind them. As she screams out for help the lights in the corridor outside go out.

Now inside the house, Donna talks with Wilf about her travels. He seems excited for her but is concerned for her safety. Donna claims the Doctor will look after her then turns her attentions to her mother Sylvia, whom Wilf claims must not be told of her adventures because she will not condone them. Despite feeling torn on the matter Donna remains tight-lipped when Sylvia enters the room.

On their way to see Luke, Private Ross tells the Doctor that UNIT has been watching the Rattigan Academy for some time – having become concerned by the ‘Hitler Youth’-style regime the students are subjected to. The Doctor notes that their jeep is equipped with an ATMOS device, which Ross explains is standard issue on all company vehicles.

Eventually they arrive, and Luke is waiting for them. He claims Colonel Mace has notified him of their arrival then follows the Doctor as he runs inside. He excitedly examines the science experiments being conducted by the students – which centre on developing the ability to terraform an entire planet.

They move into another room where the Doctor confronts Luke about the difficulties of being a genius. He does, however, claim the young man is not responsible for creating ATMOS as the technology is too advanced. He then notes another anomaly – a large purple teleport pod in the corner of the room. He steps inside and activates it.

Arriving in a large chamber drenched in purple light, the Time Lord finds he is not alone – dozens of Sontaran soldiers also occupy the room. Upon seeing the intruder, they advance towards him but before they can stop him the Doctor manages to teleport back to the Academy, where he warns Ross and Luke that they must leave. However, it is to late; General Staal arrives and aims his gun at the Doctor and Ross. Before he can fire though, the Doctor zaps the teleport with the Sonic Screwdriver.

By displaying a level of understanding about the Sontarans that surpasses the knowledge of humans the Doctor prevents Staal from shooting him or Ross. He explains to his UNIT friend that the Sontarans are a warrior race that expands its numbers via mass cloning. He claims the creatures have one weakness – a probic vent on the back of the necks, which when hit renders them disabled.

He picks up a nearby squash racket and ball and manages to aim a shot that rebounds off of the teleport pod and hits Staal on the neck, causing him to stagger. Ross and the Doctor flee outside whilst Luke watches Staal regain his composure. The alien general then repairs the teleport with the aid of his gun and he and Luke step inside.

They arrive in the purple chamber where they are joined by Commander Skorr, who asks Luke how many ATMOS devices have been installed in vehicles on Earth. He claims 400 million cars have been converted, which is enough to activate ‘the conversion’. He further explains that the devices were tested and proved fully operational – the test having been the cause of the fifty-two simultaneous deaths the previous day. Staal asks if UNIT has been infiltrated and Skorr claims it is almost complete.

He teleports down to Earth and arrives inside the hidden factory chamber, where Martha has been strapped onto an operating table. He explains to her that the UNIT soldiers and factory staff have been controlled using hypnosis, but her use will be more complicated. As he speaks a human hand emerges from the vat of green liquid, and the Sontaran tells Martha that soon the creature is sign “will be you”.

Back on the Sontaran ship, Luke gazes down upon the Earth via an observation deck. Staal joins him and Luke claims the planet was never big enough for him – he wants something more. Staal commend him on his ambition then the two discuss the Doctor, who is an old enemy of the Sontarans. Staal tells Luke that legends claim the Doctor led the battle in the Last Great Time War, which the Sontarans were forbidden from participating in. Now he will be able to seek revenge by destroying the last of the Time Lords, along with the planet he loves so much.

In the factory laboratory Commander Skorr explains that Martha is to be cloned. Sure enough the creature inside the liquid emerges and reveals itself to be an exact replica of her. Martha protests but is lulled into sleep, unable to prevent the Sontarans from duplicating her memories and depositing them into the mind of the clone.

Elsewhere, the Doctor and Ross begin the journey back to the factory, but are deterred when the Sontarans override the ATMOS device in their jeep and order it to drive them into a nearby river. Finding all the doors deadlock sealed the Doctor is forced to interface with the device, discovering that is has been programmed to contradict his instructions. He orders it to drive the vehicle into the river and it does the opposite – it stops inches away from the water’s edge and allows the passengers to exit. They run away and duck for cover as the ATMOS device goes haywire and terminates – not with a bang but a whimper.

On the Sontaran ship Staal and Luke see the device’s signal fade and assume the Doctor to be dead. Staal then orders the other devices to be primed and ready. Back on Earth Commander Skorr briefs Martha’s clone and orders it to go about its work, accompanied by the two Sontaran-controlled soldiers.

The Doctor and Ross arrive at Donna’s home and together they inspect Sylvia’s car, which is fitted with ATMOS. Wilf arrives to greet the Doctor and the two realise they have met before, on Christmas Eve when the Doctor briefly visited Earth whilst attending a party on the Starship Titanic.

The Doctor then calls Martha to warn her that UNIT must prepare for the Sontaran invasion, but must not open fire because their troops will be massacred. Unaware that he is speaking to a clone the Time Lord assumes his instructions will be carried out, but in reality Martha’s double disregards the warning and tells Colonel Mace there has been no cause for alarm.

The Doctor tries to disarm the ATMOS device attached to the engine of Sylvia’s car. He realises the navigation apparatus is hiding another in-built system but he is unable to access it. Sylvia then arrives and it horrified to see the Doctor again, still appalled by the destruction he caused at Donna’s wedding reception. She is cut off when the Doctor manages to trigger the hidden weapon built into the ATMOS device, causing smoke to pour from the engine. A signal is sent to the Sontaran ship warning them that one of their converters has been activated, which leads Staal to realise that the Doctor must still be alive for only he could have done this.

The Doctor manages to stem the flow of the gas coming from the triggered converter and realises that the fumes contain a form of poison. Wilf clambers inside the car in the hope of driving it out of harm’s way, but is trapped inside when the ATMOS device reactivates and deadlocks the car doors.

Up above the Earth, Staal orders for every converter to be activated. As hoards of Sontaran soldiers prepare to swarm the planet those left on the ship rejoice at their impending victory, and chant the Sontaran battle cry.

Down below the UNIT soldiers hurry to try and prevent the unfolding chaos, whilst Martha’s clone and the conditioned guards look on calmly. All over the world cars begin emitting the toxic gas into the atmosphere, and no one is able to stop it.

Meanwhile, Donna tries desperately to free her grandfather, who is becoming engulfed in the poisonous emissions from the car. As Sylvia hurries back inside the house the Doctor is left to simply look on as the streets around him are buried beneath the deadly fog…

As fumes engulf the Earth the Doctor struggles desperately to free Wilf. His efforts are trumped though, when Sylvia arrives with a fire axe and smashes the windscreen.

At the ATMOS factory Colonel Mace orders his men to seal off all doors to prevent the gas from getting inside. He returns to the mobile HQ where his technical staff tells him that ATMOS has been activated across the entire world. Meanwhile, Martha’s clone enters and begins accessing the NATO defence system.

At Donna’s house Sylvia leads the freed Wilf inside. Seconds later Private Ross arrives in a taxi (the only vehicles not fitted with ATMOS) and tells the Doctor to get inside. Despite Sylvia’s protests Donna also goes with them, and finds support in Wilf, who realises she will be safer with the Doctor.

Back at UNIT mobile HQ Clone Martha downloads NATO security protocols onto a data chip and loads it onto a mobile phone. The codes are transmitted up to the Sontaran battleship, where General Staal orders that she must be protected from being discovered. Commander Skorr agrees to launch an attack squad in the factory and leaves. General Staal then turns to Luke, who is rejoicing in the destruction on the planet far below.

Whilst news reports warn the public to stay away from the poisonous gas, the Doctor, Donna and Ross return to the ATMOS factory. Inside the mobile HQ Clone Martha delivers the Doctor’s previous instructions to Colonel Mace and warns him that they are facing a threat from the Sontarans, a race UNIT has encountered before. Outside the Doctor tells Donna to return to the TARDIS where she will be safe, and hands her a key to the ship. He then hurries back inside.

He arrives in the mobile HQ and warns Colonel Mace not to engage the Sontarans in battle because the human troops will be slaughtered. He tells him that he will use the TARDIS to go onboard the Sontaran ship and prepares to leave, seemingly unaware that Clone Martha has sent a signal to the two conditioned UNIT soldiers waiting outside.

As the impostor companion leaves with the Doctor the two soldiers approach the nearby TARDIS and attach two alien devices to its doors. The Sontarans activate the pieces of equipment and the ship is teleported onto the battleship, with Donna still inside. General Staal examines the machine and is jubilant it is now his. Luke Rattigan, meanwhile, returns to Earth to summon his student and ready themselves for battle.

The Doctor and Clone Martha arrive in the alleyway where the TARDIS was taken and the Time Lord realise sit must have been taken by the Sontarans. He begins to question his companion; surprised she has not mentioned any concern for her family. She bluffs her way out of the situation and then returns with the Doctor to the ATMOS factory.

The students of the Rattigan academy watch as London is engulfed in fog. Luke arrives and tells them that they must not concern themselves with it because they must concentrate on their work.

Returning to HQ the Doctor listens as the first reports of death from the fumes are sent in. Meanwhile Colonel Mace has been preparing to fire NATO nuclear strike at the Sontaran ship, which has been detected orbiting the Earth. The Doctor warns him such an attack will have no effect on Sontaran defences and instead opts to talk to the creatures himself.

He establishes a visual link with the battleship, which is overseen by Donna in the TARDIS via the scanner. However, unbeknownst to Donna that is not the only signal reaching the ship. Before the image of the Doctor appears the face of a girl screaming for help is seen amidst the interference on screen – it is Rose.

The Doctor turns on General Staal, calling him a coward for using poisonous gas instead of confronting the humans face-to-face in battle, as would befit a Sontaran. He theorises that instead they must be planning something else; this is not usual Sontaran behaviour. He questions whether it is a strategic move in the Sontaran’s war with the Rutans, which he suggests may be going badly for the aforementioned side. Staal rubbishes this claim and leads his men in a victory chant, which the Doctor ignores. His attention is gained, however, when Staal reveals the TARDIS is onboard his ship.

The Doctor manages to speak with Donna without arousing suspicion from Staal, and tells her to call him on the mobile phone he keeps on the console. She finds the device but doesn’t know what number to call. Meanwhile Staal severs the communications link when the Doctor threatens to regain the TARDIS using the Sonic Screwdriver as a remote control device. The ship is moved out of the main war room, instead being placed in a section of the ship where the transmission field from Earth will not reach it.

At the Rattigan Academy Luke tells his students that the experiments they have been conducting are preparations to be used to create a new world for them to live on. The Sontarans have promised to take Luke and his students to a new world where they can create a new superior human race, away from the scorn of people who belittled their intelligence. However, the young scientists do not believe in his plans, for they are more concerned with the well being of their families. They desert him, and he is left in a fit of rage.

The toxic fumes continue to spread and the population descends into panic. Thousands of people begin fleeing into the open countryside whilst others are seeking refuge at sea.

In the TARDIS Donna calls her mother and tries to reassure her that she is safe and everything will be all right. Wilf takes the phone and speaks to his granddaughter, concerned that she is safe from the fumes. He is disgruntled that the Doctor is not with her but seems assured when Donna promises that wherever he is, he will be able to save them all.

At UNIT mobile HQ Clone Martha has analysed a sample of the gas and shows the report to the Doctor. He notes that there is an unidentified element in the chemical composition, and that it is the rogue element that the Sontarans need. He believes the gas is not just poison; it is something else as well…

Colonel Mace begins to launch the nuclear strike and the Doctor is distraught. As each nuclear station across the world goes online General Stall looks on from the Sontaran battleship, claiming, “the bravery of idiots is bravery nonetheless”. Commander Skorr claims their operative working within UNIT will be able to stop the attack, and then assures his General that the Sontaran Attack Squad is ready.

As the countdown to nuclear strike reaches zero Clone Martha is able to stop the missiles using the protocols she has downloaded. The Doctor comments that the strike wouldn’t dent the battleship yet the Sontarans are keen to stop the attack. He turns to Martha for an explanation but she has none.

Soon the Sontarans are beginning to swarm the lower levels of the factory, with the objective of collecting the Clone of Martha. Private Ross and his men hear the troops approaching and prepare to defend themselves, but their weapons are rendered useless by the alien signal being transmitted by the Sontarans that causes the bullets to lodge in the barrels of the guns. Ross contacts Colonel Mace to warn him of the attack and the base goes into red alert. He and his men are gunned down, and the Sontarans advance.

The Doctor orders Colonel Mace to withdraw his men and he complies, ordering a full retreat. However, battle has already begun as the Sontarans storm the factory. The two conditioned guards are gunned down in the process as the UNIT soldiers are massacred, just as the Doctor predicted. The Time Lord wonders why the factory is so important to the Sontarans and wishes he had the Brigadier with him to help. Colonel Mace apologises, claiming Sir Alistair is stranded in Peru.

Another nuclear strike is prepared but again Clone Martha manages to thwart it. Colonel Mace order the source of the sabotage is found, whilst outside the toxicity of the gas increases.

Luke returns to the Sontaran battleship, claiming he has failed in his mission to gather the population of his new planet. General Staal seems nonplussed by this, and admits that it doesn’t matter; the students were to be used as target practise in any instance. Luke is confused by this and listens as Staal explains that the promise of a new world for Luke was simply a ploy to secure his cooperation – he was needed simply to install the ATMOS system – nothing more. Luke grows infuriated by this but evades being shot down by teleporting away. Staal brands him a coward and then orders for all teleport links with the ship to be closed. Back on Earth Luke is left alone in his Academy, reduced to tears.

At UNIT mobile HQ the Doctor continues to wander what it is that the Sontarans need inside the factory. As Colonel Mace continues in vain to try and fight back the Doctor takes Martha’s phone and calls Donna. He explains that he needs her to reopen the teleport link, which he has assumed will be closed down now the creatures are at battle stations.

Reluctantly she agrees and slowly opens the TARDIS door, only to find a Sontaran standing guard outside. The Doctor tells her to hit the creature on its probic vent, which she does so using a hammer sitting beside the ship’s console. She progresses down a corridor and the Doctor tells her to find the teleport controls. He guides her along but is forced to abandon her when Colonel Mace arrives. He hands the Doctor and Clone Martha gas masks, and they follow him outside.

Meanwhile, on the Sontaran ship, Donna takes cover as a troop of soldiers pass her in the corridor. She sits and waits for them to pass; terrified that she might be seen.

Colonel Mace shows the Doctor a new supply of ammunition which should be unaffected by the Sontaran’s crippling signal. The Doctor is still sceptical and notes that battle will be useless in the fog – they cannot see the enemy. Mace ignores him and instead rallies his troops to fight back. Seconds later a strong wind picks up and the fog begins to clear. The Doctor looks up into the skies and sees a familiar shape – the Valiant. Now a UNIT carrier ship the craft’s engines are powerful enough to sweep away the gas, leaving the air clear.

Mace then orders the ship to fire upon the factory. A lightening green laser shoots from the ship and hits the complex, demolishing the Sontaran forces inside. Soon the UNIT soldiers are putting up a palpable defence and the troops begin to swarm the building. The Doctor and Clone Martha hurry inside, the Time Lord taking a moment to assure Donna via the phone that he is still with her. He begins scanning for alien technology and heads towards the basement. Martha is in quick succession, pausing only to stop another nuclear attack.

Eventually the duo arrives inside the chamber where Martha was cloned. The Doctor discovers the real Martha still strapped to the operating table, and runs to help her. He confesses that the clone of his friend did not convince him but he found her useful for stopping the nuclear strikes. He realises that the reason the Sontarans protected the factory was to stop the real Martha from being discovered – whilst she is unconscious the clone can survive.

He disconnects Martha from the Sontaran equipment and immediately her clone topples over in agony. The Doctor helps Martha as he reawakens then answers a call from Donna, who has found the teleport controls on the Sontaran ship.

Whilst Colonel Mace dispatches Commander Skorr up above, in the basement the Doctor tries to repair the teleport unit and Martha confronts her duplicate. She tries to ascertain what the Sontarans are using the gas outside for and she recites the unknown part of the toxin’s chemical composition. The Doctor recognises the new element as clone feed, and realises that the Sontarans are going to use Earth as a clone planet. They have no need to invade because soon new warriors will be born all over the planet.

The beat of Clone Martha’s heart begins to slow, and the real Martha is only able to look on as her duplicate dies. She takes the wedding ring from the clone’s finger and places it on her own.

Donna calls the Doctor and tells him she has completed her work, but the Sontarans have found her. They aim their weapons at her but before they can fire the Doctor manages to bring her back to Earth. He then sends the TARDIS back to the alleyway and ushers Donna and Martha inside the teleport pod. He activates the device and they arrive at the Rattigan Academy, where the distraught Luke is still alone.

On the Sontaran ship General Staal orders the ATMOS devices to be set to maximum conversion, and across the globe the fog gets thicker. At Donna’s home Wilf and Sylvia find themselves at the mercy of the gas, which is seeping into the house. Sylvia breaks down in tears and Wilf comforts her, reminding her that Donna promised they would all be saved.

At the Academy the Doctor rushes around trying to construct one of the experiments Luke’s students were working on. He explains to the others that the Sontarans were holding back the nuclear strikes because the missiles would ignite the atmosphere and burn away the gas. Now he intends to finish the job only by a more humane means – by using an atmospheric converter made by the students.

General Staal prepares to launch the clone pods that shall soon inhabit the Earth whilst on the planet below the Doctor runs outside and activates the atmospheric converter. A bolt of light fires up into the sky and ignites the poisonous fumes. Soon a blanket of fire is sweeping across the city. Staal can only look on in vain as the purifying flames spread across the Earth, leaving the air breathable once more. The UNIT troops and the people of the Earth rejoice, just as Donna promised, they have been saved.

However, the Doctor claims there is still trouble to come, and he is right. On the battleship General Staal orders his army to prepare for an invasion – the Sontarans cannot accept defeat and so therefore they shall wipe out the human race as compensation for failing to create a new breeding planet.

The Doctor hurries inside the Academy ad steps inside the teleport pod. He says goodbye to Martha and Donna and then tells Luke to do something clever with his life. Martha asks why he is saying farewell and he explains that he must stop the Sontarans – which he will achieve by destroying their ship with the recalibrated atmospheric converter. Martha asks why he cannot send the device on its own and set it on a timed delay, and he tells her it is because he must offer the Sontarans the option of surrender.

He teleports away and arrives on the ship, where he tells General Staal that if the Sontarans leave he will not activate the device and destroy him. Staal tells him the Sontarans do not fear death, and his warriors aim their guns at the Doctor. As the battleship prepares to open fire on the Earth the Doctor hesitates, but his choice is made for him when Luke reactivates the teleport.

He sends the Doctor back to Earth and takes control of the atmospheric converter’s controls. With a mocking recitation of the Sontaran battle cry he activates the device and the Sontaran battleship is destroyed in a colossal explosion.

The city of London returns to normal, only now the public are somewhat reluctant to use their cars. Donna returns home again to see her family, and once more Wilf warns that her mother must not know of her travels with the Doctor. He tells he she must go with the Time Lord again and see the stars, and tearfully she says goodbye to him.

Eventually she returns to the TARDIS, where the Doctor and Martha are waiting for her. She asks Martha is she going to join them on their next journey but she declines the offer; although she has missed her time with the Doctor she has a whole new life to explore on Earth.

She is about to leave when the ship doors slam shut and the console begins to set the craft in motion. The three travellers are left at the mercy of the machine, which appears to be piloting itself. As the jar containing the Doctor’s severed hand begins to bubble and glow Martha orders the Doctor to take her home…