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:
“I learned a long time ago that if you’re missing somebody, just look up at the night sky. Whoever it is, wherever they are, chances are they’re looking at the stars just like you. Sometimes for all its size the universe isn’t such a big place after all.”