On the first day of the new school year, Maria and Luke arrive in the courtyard, Sarah Jane kissing her new son before she departs, asking him not to caller her “mum”. The two friends then walk into their new school, admiring a new building standing before them.

Watching them from somewhere within the building, the Headmaster of the School (Mr Blakeman) observes them, before focusing his unseen camera on Mr Jeffrey, a science teacher.

Moments later he is leading him through a corridor of the new building, positioning him by a blank wall. Suddenly it rolls back and a giant green arm reaches from within, grabbing Mr Jeffrey.

In the school hall Maria and Luke are met by Clyde, another new student. Somewhat bemused by Luke’s overly formal handshake, he introduces himself as the room falls silent. Mr Blakeman enters, and after breaking wind he rushes through the formalities of ringing in the new school year, Mr Jeffrey joining him on stage part way through.

He explains the new building is the technology block, and some time later he is giving Maria and Luke’s class a guided tour through the building, Clyde noting the strange smell around them, a smell like batteries.

Later, in the canteen, Clyde joins Maria at her table and the two share stories of their parent’s separation. They are shocked by the mouldy food they find on the plates before them, Clyde claiming something weird is going on in the school.

At the end of the day Mr Blakeman and Mr Jeffrey watch the children go before walking off to “test” their new building.

In Bannaman Road, Maria’s father watches as his daughter, Luke and Sarah Jane arrive back from the school in Sarah’s car, and he teases her about not walking home instead. They joke about the issue as Sarah looks on in admiration at their relationship. Once Maria and Luke have gone the two parents talk about the new technology block, Mr Jackson remarking that he helped construct a similar building elsewhere in London; it too had an odd metallic smell within.

Back at the school the two teachers rush about a darkened control room, pulling levers on giant control panels. Electricity buzzes around them and they look around with glee.

In her attic, Sarah talks to Luke about his first day at school, reassuring him that he must take time to adapt to a new life, he may be different but he must live a normal human life. Conversation turns to the new technology block and Sarah reveals she has been investigating it’s construction firm: Coldfire, who have been building identical buildings all around London.

“Makes a change for me” Sarah claims, “Not aliens is it?”

At the school the two teachers continue their work, whilst at Maria’s house her mother arrives to investigate the new house and collect a duvet from her ex-husband.

Meanwhile, the teachers conclude their work and Mr Blakeman claims with relish: “Lights out, London”

Sure enough the lights in Maria’s house all go out. Maria fetches a torch from the cupboard but it too fails to illuminate, apparently from a faulty battery. Across the road the power in Sarah’s attic has gone as well, even Sarah’s specially adapted watch has gone dead.

Maria’s father fetches candles to illuminate the room, but his attempts to light them are of no avail, they die out as soon as he ignites the wick.

In the school the machinery begins to destabilise and Mr Blakeman deactivates it, returning power to the city. The candles light up, the torch turns on and the electricity all comes back to life in Bannerman road.

Mr Jeffrey chides Mr Blakeman for his failure but he is adamant he will manage to correct the fault, one more adjustment and they can destroy the planet…

The next day Sarah leads Luke outside before he starts his walk to school. Sarah reassures him again that he will be okay before moving to talk to Mr Jackson, who shows her the plans to the school he worked in. She finds a blank space on the map, which appears not to have a door.

She arrives at the other school sometime later, whereupon the headmistress explains to her that ever since the new block was installed food has been going off and an odd smell had been lingering in the corridors. Sarah continues to look around; unaware she is being watched via a security camera.

After impressing Mr Jeffrey in his science class, Luke attends Science Club at lunchtime, finding himself and a boy named Carl the only ones in attendance. Mr Jeffrey introduces the circuit diagrams for his and Mr Blakeman’s machine, not explaining its actual purpose. As he hoped, Luke finds the fault in the circuit and writes down a solution.

Sarah makes her way to Coldfire construction, phoning Maria on her way and telling her to investigate the new block to try and find the source of the smell.

At the end of the school day as Mr Blakeman and Mr Jeffrey rejoice at the solution to their problems, Maria, Luke and Clyde begin to scout around, whilst at Coldfire Construction Sarah is greeted by a company secretary.

At the school, Luke realises that the dimensions of the block do not measure up, there is an empty space concealed behind one of the walls.

Sarah interviews the secretary, called Janine, and reveals she has found that Coldfire have been constructing buildings in cities all over the world, and each one has been surrounded by rumours of food going stale and strange smells lingering nearby.

Maria arrives in one of the computer rooms, but Mr Jeffrey is soon on her tail. She hides under a table but he can still smell her presence. He claims that he need no longer hide and proceeds to pull open a zip on his forehead, peeling away his skin to reveal a giant green alien hiding inside.

“I am Slitheen”

Maria flees, whilst at Coldfire construction Janine proceeds to do the same, causing Sarah to flee her office, closely followed by another of the creatures. It follows her into a storage room, where the reporter hides behind a stack of boxes. However, the smell of her perfume is enough to give her away, and the revealed Slitheen advances on her.

At the school, Maria finds Clyde waiting in the corridor, Luke nowhere to be seen. The Slitheen advances on them and they run, whilst Luke manages to locate the entrance to the hidden room, and steps inside. Looking around in amazement he turns to find Mr Blakeman waiting for him.

Cornered below some stairs, Clyde and Maria are found by Carl, who leads them into another room. He closes the door before revealing there is no way out. He reaches up to his forehead and peals away his skin, revealing a far smaller but equally disturbing creature inside.

Clyde and Maria run, whilst Luke manages to dodge Mr Blakeman and escape from the hidden control room. At Coldfire construction Sarah Jane distracts Janine Slitheen by spraying her with her perfume, and manages to run back to her car.

Meeting his two friends in the foyer of the building, Luke watches as Carl Slitheen advances on them. Maria flees down a corridor as Luke and Clyde run up the staircase. Sarah Jane then calls Luke and tells him to distract the creatures by distracting them with a smell.

As the young Slitheen and its father advance Clyde pulls out a deodorant aerosol from his bag and sprays them, giving them time to return downstairs. Maria joins them as Sarah Jane arrives outside. She uses her Sonic Lipstick to open the locked doors and the three children join her outside. Before the Slitheen can escape Sarah seals the doors once more and the friends drive away.

Carl Slitheen complains that his hunt has been disrupted whilst Mr Blakeman and his accomplice, the unmasked Mr Jeffrey, theorise what authority Sarah Jane may hold considering her possession of advanced technology. Again the young Slitheen complains but his father comforts him, he will soon get his hunt; “After all, tonight’s the night the lights go out!”

Arriving back at Sarah’s house Maria warns Clyde he must forget what has happened and return home. However, after he threatens to call the police she allows him to come inside. As Mr Blakeman and his accomplice rejoice in their upcoming victory; the chance to regain fortunes and return to their homeworld, Sarah begins to theorise what the creatures are doing on Earth.

She and Luke discover, using Sarah’s special watch, that the Slitheen are a family of scavengers who hide themselves by using the skins on native species. Suddenly Clyde bursts in with Maria, but Sarah ignores his protests to continue her deductions. Luke explains about the secret room but requires help to sort out his own theories.

Sarah calls for Mr Smith, and after she does so a large section of the wall of the attic slides away, revealing a giant computer. It shows them a satellite picture of London, with a series of dots showing the locations of buildings constructed by Coldfire. Sarah then asks to see all buildings constructed by Coldfire around the world, and she realises that they only exist in cities that have an underground railway system.

The ten cities form a loop around the whole globe, with a Slitheen posted at each location. Luke then realises that in helping Mr Blakeman he has made a terrible mistake; “I told the Slitheen how to destroy the Earth.”

At the school Mr Blakeman inputs Luke’s equation into the computer before telling all of the other Slitheen to stand by.

In the attic Mr Smith explains that the buildings are conductors, when they are all working they transform light and heat into electrical energy, as they did the previous night when Bannerman road was hit by a blackout. Sarah then realises that the underground railways act as a cooling system for the heat given off as a side effect, and the rotten food is a result of being near to the conductors.

Maria notes that if they can take out one of the conductors the chain will be broken and the system will fail. The Sonic Lipstick will be able to put the machinery out of action, but they have no way of tackling the Slitheen.

As news reports come in about power losses around the globe, Mr Blakeman orchestrates the operation from the school, each continent falling silent as the blackout approaches Britain.

Sarah asks Mr Smith to provide her with all possible information concerning the Slitheen, but before he can identify their weaknesses the giant computer cuts out; the power drain has started; even the Sonic Lipstick has been rendered ineffective.

The four friends prepare to leave for the school again, but before they go Clyde remembers how Mr Jeffrey reacted when he found his homemade chip-sandwich in his bag that morning. They determine that it was the vinegar inside the sandwich that caused him to recoil, acetic acid causing a reaction with the calcium in the Slitheen’s bodies. They then run to the kitchen, finding all the vinegar they can and pouring it into spray-bottles.

Now armed, they run through the streets towards the school, whilst inside the building the Slitheen put the final preparations in place, finally using their transformer to drain the power from the sun. As the air turns cold and the sky fades into dark, Maria’s mother arrives at her home, distraught that her daughter is nowhere to be seen.

Soon Mr Blakeman and the others detect Sarah Jane and the others arriving at the school, and the headmaster decides it is time he dealt with the intruders. He unzips his skin to reveal the Slitheen within. He finds them in one of the corridors and advances on them, but they use the vinegar to hold him off. Sarah and Luke run off to find the control room as Maria and Clyde are left to fend for themselves.

Blakeman taunts Maria for not having the nerve to use the vinegar to any great effect, and insults her along with the rest of the human race. However, his heckles give her the determination to act and she showers him in the liquid, causing him to explode in a pool of slime that showers the two jubilant schoolchildren.

The other Slitheen sense something is wrong and warns the rest of the family that something is wrong. They teleport from around the world to join them as Sarah and Luke arrive. The Slitheen hold them as Mr Jeffrey explains their plan; to suck the energy from the Earth until the atmosphere collapses, leaving the planet dead but them alive and well, once they are collected by a spaceship they will sell the stored energy contained within the converters.

Sarah asks why they would do such a thing and the Slitheen explain that the other members of their family who once visited the Earth were destroyed, they now want to take revenge, and Luke has given them what they needed.

However, Luke reveals that he did not know their plan involved taking energy from the sun, the equation he gave them will not work. Sure enough the machinery begins to overload and as Maria and Clyde arrive (only to be held as prisoners along with Sarah), the creatures deactivate the transformer.

Luke agrees to fix the problem, whilst outside the sun returns to normal. Inside the lab Sarah throws the reactivated Sonic Lipstick to Luke, but a Slitheen is soon on his tail. He manages to evade capture and zap the controls with the device, causing the system to overload. All but two of the Slitheen manage to teleport away, and the last two, who become trapped inside the secret room, beg Sarah to save them from the blast.

She look on and raises the Sonic Lipstick to free them but before she can act the young Slitheen disappears the elder is destroyed in the blast. Suddenly the lights in the building return, the system has burnt itself out.

Soon the friends are returning to Bannerman Road, Sarah contacting UNIT to ask them to disable the remaining Coldfire buildings. Maria’s parents greet her and she returns to her home, where her mother agrees to stay for dinner. She recalls the strange events that have occurred since Maria moved house to live near Sarah Jane, but the only connection she makes between the reporter and the odd goings on is that she herself has been affected by everything.

In Sarah’s house, Clyde questions his new friend about her knowledge of aliens. She tells him of the Doctor, an alien whom she travelled with through time and space and whose legacy she carries on in her life to the present day. He then tells her than Luke, her son, shouldn’t have to call her “Sarah Jane” anymore, he needs a “mum”.

Up in the attic Clyde tries to question Mr Smith about his presence in Sarah’s home, but he and the others are more concerned with covering up the recent events in the media. Maria then takes her excited new companion to catch a bus home, leaving Sarah and Luke alone.