In the attic of 13 Bannerman Road, Sarah and Maria look out at the night sky. Sarah recalls how she used to stargaze as a child and Maria remembers the adventures they have shared together.

“And there’s still so much more to discover...”

Meanwhile, in an observatory some miles away, Professor Skinner and his daughter Lucy watch the constellations and solar systems together, discussing Lucy’s future. Soon they discover a wave pattern is being picked up by their sensors, which is followed by a noise outside.

Back on Bannerman Road, Maria looks through the morning post and finds a letter for her father Alan, which she assumes to be the response to his application for a new job in London. He opens the envelope.

In Sarah’s attic Luke and Clyde are playing computer games on Mr Smith when Sarah enters. Addressing her computer, she enquires about a village named Goblin’s Copse, whose residents have reported seeing strange lights in the sky.

The letter Alan has received is not a job offer for London – but what it does entail will have far-reaching consequences for Maria and Sarah Jane. After telling her father she supports whichever decision he makes, Maria crosses the road to find Sarah, Luke and Clyde about to take a ride into the country. Sarah invites her along, and she decides she will join them, whilst she still can...

Eventually they arrive at the radio telescope operated by Professor Skinner. Clyde expresses his dislike for the place but joins the others in venturing inside, unaware they are being watched by something hiding in the woods.

Sarah and her friends find the interior of the observatory deserted, and after searching the data records Luke discovers an irregularity had occurred the previous night at 10 o’clock, the same time the villagers saw lights in the sky.

Suddenly Lucy runs in, gasping for breath and claiming there is something in the woods. When she eventually calms down she asks if any of the visitors has seen her father, but they tell her they have not. She recalls the events of the previous night, when her father disappeared and something chased her through the woods, causing her to trip and bang her head. Sarah asks what it was she saw in the forest and she claims it did not appear to have a physical presence – as if it were invisible.

Clyde decides to search the woods and takes Luke with him but before they can leave Sarah orders them to stay put. Clyde protests, claiming Professor Skinner may still be hurt, and Sarah agrees to let the two boys search for him, but only around the observatory buildings. They leave and immediately Clyde heads for the trees, whilst the creature watching him and Luke manages to make itself invisible.

Inside the lab, with Lucy now asleep, Sarah and Maria are left alone. At last Maria reveals what occurred that morning – her father has been offered a job in Washington, America. Immediately Sarah seems to turn cold, and accepts without question that Maria must go.

On Bannerman Road Alan tells his estranged wife Chrissie about his new job, and she seems alarmed that her daughter might be taken away from her. Her ex-husband seems to share her concerns, explaining that the move will mean more upheaval for Maria than it will for him.

Moving deeper into the woods, Clyde and Luke begin to feel like they are being watched, and Luke senses an electrostatic field nearby. Suddenly something moves behind them and the boys turn, but the creature does not attack – it is studying them. Eventually the stalker reveals itself – a Sontaran.

A chase ensues, but Luke and Clyde manage to evade their alien pursuer. Soon they find themselves lost, emerging in a more open area which appears to be empty. However, they soon discover that another invisible object has been hidden in the forest, only this time it is not a living thing.

When Lucy awakens she hurries to find her father but Sarah manages to stall her and asks about the lights that appeared the previous night. She and Maria listen as Lucy explains that the lights were the size of footballs, and Sarah theorises they might be drones for something bigger.

The girls are interrupted when Professor Skinner returns. Lucy embraces him but his response is not as warm as it once was. Lucy recalls sensing something in the woods but he flatly denies there is anything there. Sarah cuts in and tries to interview him but he fobs off the phenomena he and Lucy encountered as ball lightening, not alien ships.

Leaving the observatory, Sarah receives a phone call from Luke. Inside the lab Professor Skinner resumes his work. When Lucy tries to talk to him he gets up from his seat and walks quietly towards her. She backs away and then trips over.

Sarah and Maria drive into the woods to collect Clyde and Luke. They take them to the object they have found, which Luke believes to be camouflaged. Clyde feels his way around what they now believe to be an alien ship and claims it is round. Sarah decides to find out by cancelling the perception filter with her Sonic Lipstick. When she sees the ship in its true form she urges the group to leave, and claims she must call in UNIT.

She stops only to explain that the ship belongs to a Sontaran, a race she hoped never to encounter again, having met them twice before. She tells Luke, Maria and Clyde that the Sontarans are ruthless and brutal, and have only one motivation – conquest.

Suddenly the Sontaran who owns the ship arrives. Sarah confronts the creature, who reveals its name to be Commander Kaagh, of the 10th Sontaran Fleet, the one responsible for the lights seen by the Skinners last night – a lure to get them away from the observatory. He claims that soon he will avenge his people but before he can say any more Maria creates a diversion and she and her friends run off in different directions, Sarah and Clyde in one, Luke and Maria in the other.

As Kaagh gives chase, Sarah tells Clyde they must return to the observatory, convinced the Sontaran has something to do with Professor Skinner’s strange behaviour. Sure enough, when they return they discover a placid Professor Skinner still working, only now they can see that he has a neural control stuck to the back of his neck. They watch as he types away at the computer, and then notice that Lucy has disappeared. Sarah moves to disarm the neural control with her Sonic Lipstick but Kaagh arrives to intervene.

Whilst Luke and Maria approach the observatory from the woods and discover a hidden underground tunnel that will allow them to get inside unnoticed, Sarah turns on Commander Kaagh and talks about her encounters with his race, both in the past and in the future. He commends her for having met the Sontarans and survived, but Sarah is not interested in flattery. Having realised that Professor Skinner is hacking into the access codes of various satellites orbiting the Earth, she demands to know what Kaagh is planning.

He explains that he is the only survivor of the Sontaran Invasion Force that tried to take over the planet using ATMOS devices to poison the atmosphere. However, their plans were thwarted by a man known as the Doctor. When the Sontaran Mothership was destroyed, Kaagh was already on his way to Earth to assist the ground forces, but the explosion sent him spiralling into a crash landing. He was left abandoned on Earth, the sole survivor of a campaign thwarted by an unarmed man. Determined to wreak revenge, he repaired his ship and began setting about enacting his plan, which he claims will bring honour to his shamed name, and make him a hero on his home world Sontar.

As Maria and Luke arrive, listening from a safe distance so that they are not spotted, Kaagh reveals that he intends to bring the thousands of satellites orbiting the Earth crashing down into the nuclear reactors populating the planet, thus burning it to a cinder. As soon as the satellites reach their primary alignment the radio telescope of the laboratory will send a signal to bring them down. The alignment will be reached in forty-five minutes. The countdown has begun.

Sarah falls to the ground, not dead but stunned. Kaagh grabs Clyde and tells him she will be taken back to Sontar to pay for the Doctor’s crimes. He orders Professor Skinner to tie Sarah up then begins theorising what will be done with Clyde – explaining that he has a laboratory onboard his ship, which he will use to experiment on him.

However, Maria and Luke come to the rescue, revealing their hiding place in a service shaft. Clyde escapes from Kaagh, runs to join his friends, and soon all three of them are running through the dark concrete passageways beneath the observatory. Kaagh gives chase, opening fire on them but failing to hit anyone.

Eventually the kids arrive at a heavy metal door, which they struggle to open. Kaagh arrives but is too late; Maria, Luke and Clyde have already gone through the door and barricaded it, now finding themselves back in the woods.

Inside, Sarah awakens and finds herself locked in a room with Lucy, who is concerned about his father’s behaviour. Sarah realises she has left her bag – and therefore her Sonic Lipstick – in the main chamber so cannot open the door. She turns to Lucy and tells her they will have to think of another way out.

On Bannerman Road, Chrissie and Alan continue to discuss the job offer Alan has received. Chrissie claims there is no question he will accept it and move to America with Maria, as he is driven by his work. However, Alan tells her his devotion to work came as a result of wanting to look after her and Maria.

Alan then receives a call from Maria, who explains that Sarah has been captured and in forty minutes the world will be destroyed; unless he can help her talk to Mr Smith.

In the observatory, Sarah sets about constructing a jamming device to block the radio telescope’s transmissions. Lucy voices her concern – claiming they will be traced if they do so. However, this is all part of Sarah’s plan.

After bluffing Chrissie to get her to leave, Alan runs up to Sarah’s attic and summons Mr Smith. Unaware that Chrissie has not left, and is in fact sneaking into the house via an open window; Alan calls Maria and relays Mr Smith’s information about Sontarans, which details their one weakness – a vent on the back of their necks.

Before he can continue, he is interrupted by Chrissie, who has overheard what has been said and demands answers from Alan, who is forced to terminate his phone call. Whilst Maria, Luke and Clyde hurry to find Sarah Jane, Alan takes Chrissie back outside and at last he reveals the truth about Maria and Sarah – that they hunt aliens. Knowing when Alan is lying – and seeing that this time he is being honest, Chrissie trusts his words and then hurries with him to save Maria.

Back in the woods, Clyde, Luke and Maria return to Kaagh’s ship, where Maria explains that Luke can use Kaagh’s laboratory to synthesise a knock-out gas and put Kaagh out of action.

Professor Skinner finishes networking Earth’s satellites, and Kaagh laments that in twenty-five minutes he will be able to avenge his people. He then receives a signal from his ship – an alarm warning him that Maria and the boys have gained entry.

Also aware the alarm has been triggered, Luke begins his work on the knock-out gas, utilising knowledge of alien chemicals he has gained from Mr Smith. Clyde hurries away to check if Kaagh is approaching, whilst back at the observatory Sarah and Lucy complete their jamming device, which they then activate. Detecting the interference, Professor Skinner begins scanning for the source.

In the woods, Maria finally tells Luke about her dad’s job in America and the move it will entail. Luke is visibly distressed by this but Maria urges him to continue his work – time is running out.

With Kaagh distracted by a chase after Clyde, Professor Skinner is left to find the jamming device. When he enters the room in which Sarah and Lucy are being held, they manage to escape through the now-unlocked door and trap him inside instead.

As Luke finishes formulating the gas, Clyde returns to the observatory, where he is greeted by Sarah and Lucy. Having seen her father work with the various computer systems before, Lucy begins trying to break the code guarding Kaagh’s stratagem concerning the satellites. Sarah, meanwhile, retrieves her Sonic Lipstick and goes in search of the operating system that controls the observatory’s transmissions, hoping to sabotage them.

Breaking into the observatory and turning on Skinner, Kaagh vents his fury, unaware that Alan and Chrissie have arrived in the woods, unable to find the observatory. Elsewhere in the woodland Maria and Luke approach the observatory but are chased away by the balls of light Kaagh used to distract Professor Skinner and Lucy the previous night.

Clyde and Sarah arrive in time to save them, and the four then split up. Whilst Sarah and Luke return to the observatory to deal with Kaagh, Maria and Clyde hurry to the control centre of the observatory’s satellite dish, hoping to find the main control circuit and disable it (thereby preventing the signal from being transmitted to the satellites).

Having had no luck breaking Kaagh’s code, Lucy hands over the task to Luke. With fifteen minutes left to go, Sarah ponders why Kaagh is not present to witness his moment of triumph.

In the control centre, Professor Skinner arrives and grabs Clyde, leaving Maria to find the transponder alone. Eventually she locates it, but before she can sabotage it Kaagh appears and aligns the satellite dish to carry out his plan.

As Alan continues through the forest (with Chrissie struggling to keep up with him in her high-heel shoes) Luke manages to disable Kaagh’s program. However, celebrations are short lived when Kaagh and Professor Skinner arrive, holding Maria and Clyde prisoner. Maria confesses the antenna is still operational but Sarah insists the world is safe now that Luke has disabled the code.

However, Kaagh has set a second plan in place. He activates his sleeper agent – Lucy – whom he implanted with a control device some time ago. Under his control she walks over to the control panel, but Sarah aims her Sonic Lipstick at her, hoping to deactivate the control device. Kaagh stops her by threatening to kill Maria. Sarah drops her Lipstick whilst Lucy begins reloading the code, which will immediately bring the satellites crashing down when it is complete.

Just as all looks lost, Chrissie and Alan arrive. Having heard Mr Smith talking about Sontaran weakness earlier, Chrissie jams the heel of her shoe into Kaagh’s probic vent. The electric shock that results knocks both of them to the floor. As Alan and Maria tend to Chrissie, Sarah frees Professor Skinner and Lucy from Sontaran control and she and Luke stop the code from reloading – stopping the satellites and saving the world.

Later that day, Sarah and the others return to the forest. Having deactivated the weapons on Kaagh’s ship, Sarah allows him his freedom but orders him to leave Earth. He agrees to go but warns that he will not forget Sarah, and claims that he will still have his revenge and become a hero amongst Sontarans.

After Kaagh departs in his ship, Sarah, Maria, Luke and Clyde return to Bannerman road, where Alan and Maria convince Chrissie, who has been knocked unconscious, that her encounter with Kaagh and Mr Smith was all a dream. Reunited, the family at last decide upon the move to America, and Chrissie agrees to let the two of them go.

In Sarah’s attic, Mr Smith claims there are no traces of Sontaran technology nearby; it appears Kaagh really has left Earth behind. Maria arrives to say goodbye, and Sarah apologises for her earlier behaviour – claiming she felt as if she were losing the daughter she always wanted.

Soon Alan and Maria are on their way. Luke, Clyde, Sarah and Chrissie are there to wave them off, and it is then that Chrissie reveals she still remembers what happened at the observatory. She claims she will not tell anyone else about it, noting that everyone deserves a fresh start.

That night Sarah, Luke and Clyde sit in the attic watching the stars. Sarah says: